I guess everyone, sooner or later, has a student who is a disciplinary problem. I thought I'd offer some tips for classroom discipline based on my experiences in my classes and in the psychotherapy groups I've led with children back in the States. 
My response to kids who are acting up in class is a step-by-step progressive reaction. I try to match my response to what is happening in the class right now while taking into consideration past behavior. 
Step One: Monitor your emotional status. Try to decide whether your frustration and anger are the result of stresses from outside the classroom, or if a student is provoking them. It is a sure bet that if you feel annoyed or distracted by a child, then other students do too. 
Step Two: Analyze the problem. Talk to other teachers—both the native speakers and the Koreans--to find out their impressions of the student and what they have done in response to the child. This information can be invaluable; after all, there is no sense in reinventing the wheel. Play and physical activity are important for children, and no one, child or adult, can sit still and pay attention to a lesson for 60 or 90 minutes straight without some kind of break. Plan a break or change of activity every half-hour, if not sooner. The younger the child, the shorter the attention span. The younger the child, the less ambitious your lesson plan should be.
You must analyze the pace of your class. Do the activities match the skills of the class? Is the disruptive child bored? Maybe he has mastered this point of the lesson, and is bored while you work with the other children.
Maybe he or she has not adequately understood this point of the lesson, and is bored. Check this by gently calling the child's name and if need be tapping his or her shoulder, then asking a question that will reflect comprehension. If he does not comprehend the point, probably repeating more of what you have done will not help. You may need to change tactics, or address the issue outside of class. If she does understand, however, give ample praise.
Ask questions frequently, and mix both group and individual responses. Also, make eye contact with each student throughout the class. This helps monitor pronunciation comprehension, and where the child's attention is focused. Tolerate some disruptions; they are naturally bound to occur when teaching children, in addition to teaching in a foreign culture. My main concern throughout this process is that the child masters the point of the lesson. 
Step Three: Respond judiciously to specific behaviors, both desired and undesired. Behavior can be reinforced, and bad attention feels better than no attention, so responding to minor disruptions may sometimes encourage more disruptions as a way of getting some attention from you.
Always reward desired behavior with attention, lots of attention, and avoid unnecessary or undue criticism. This goes double for your students with disciplinary problems. The rule of thumb here is "Catch 'em being good."
Disruptive behavior is usually a statement that the child is not getting something he or she needs. This may be because his or her attention has wandered, the child is bored, her toleration for stimulation is low, or she has a low level of self-discipline. Our job is to help the child return his or her attention to the lesson and to develop ways of countering the child's bad habits.
We all know disruptive behavior can be infectious, with entire classes becoming loud, boisterous, and unmanageable in a short period of time. When this happens, you must refocus the entire class. Lead by example. When the class is out of control, exhibit more control over yourself. Get the attention of some or all of the class. Sit down. Breathe deeply. Talk quietly, slowly, and with a lower voice tone.
This works well for the opposite situation, incidentally. If you want the class to be more animated or alert, talk louder and move around more. Once, I had a very sleepy early morning class of middle school students walking, then trotting, then standing, then sitting, and standing again while I was asking, "Now, what are we doing?" We did this until everyone was awake. Do not waste much time with talking quietly if it does not work immediately. Most of my classes seem to know that putting a finger to the lips and "sssh-ing" means to be quiet (they also think it is the sound of urination, but it gets their attention at the very least). The key is to get their attention and communicate the need to quiet down. An extraordinary step, I feel, is to overtly communicate your displeasure, but it has its uses. You can stamp a foot, or look crossly at the class. Koreans understand anger very well, and this will quiet the class quickly. Once the class is quiet, I hold the silence by shushing anyone who speaks or moves. After a minute or two, return to teaching as if nothing had happened. Occasionally, it is too difficult for an over-stimulated child to be still or quiet, and he or she continues to make faces or to otherwise provoke other class members. When this happens, instruct, through demonstration, everyone to put their heads down on the table. Turn out the lights, and sit in the quiet darkness for a minute or two. The most important aspect of either technique is that it gives everyone a chance to calm down--including me! Sometimes two children will aggravate each other's bad habits, and their disruptive behavior escalates. When this happens, I separate the two or three children. Sit next to the child who misbehaves. This gives them some extra attention, and helps you monitor their behavior without constantly interrupting class. Establish a time out procedure at your hagwon (or school), as Koreans do not seem to be aware of the idea. You must have a place that is safe, and not more fun than the class, where a child may wait. The child must be supervised. Allow the student to be there for two or three minutes. Set a definite time at the outset, and time it carefully. After the designated amount of time is over, go get the child. To introduce the idea to the class, I wait until someone is close to needing a time out. Then get someone to translate. I tell the child if he (usually it's a boy, isn't it?) does not settle down, he will go to time out for three minutes. The conditions for going to time out are made clear. If he tests me, I do not hesitate, or recriminate, I say in a quiet, calm voice, "time out," and escort him to the area.
Remember, no disciplinary measure is effective it if is only an empty threat. Once you have given the conditions for an action to occur, then you must follow through. Make the disciplinary steps, conditions for when they will occur, and the consequences clear to the class. This allows the child to decide his or her fate.
For the particularly troublesome kids, I make them helpers in class. They erase the board, they teach me Korean words, they run errands for me before, during (sometimes), and after class. I use English every time I talk to these kids, and they use it with me. Use your imagination when trying to find solutions to your disciplinary problems. Analyze the problem, get some help in solving it, and try something different if necessary. Perhaps we could share some of the things we have done in this regard.
I hope this helps. I encourage anyone with thoughts, criticisms, suggestions, other ideas, etc. to respond by writing to the editor, or contacting me at 012-1571-8424. Good luck!