In order to refrain some students from exhibiting off-task behaviour, I normally make eye contact with them or make some gestures such as raising my hand or putting my index finger in my mouth. Unfortunately this is not always fruitful making me utilize other sorts of interventions in order to keep the class progressing. 

Approaching the off-task or disruptive students might do the trick sometimes, yet some of them will not feel bothered by your being around and not rarely, if inquired about what they are supposed to do, they may come up with the classic “Why are you asking me instead of my colleagues?”. 

Another non-verbal cue is to make the gesture meaning ´I have two eyes on you.´ and it tends to work for a while, however, some defiant students probably will not feel troubled by this as well. 

So the next war manoeuvre is to give these fellow students choices, `It’s either your being with the group and perform the task or off you go.`. Of course this comes as the last resort but once the teacher has offered other possibilities, they may do what they promised, otherwise, the whole group may think that the teacher is not worth being respected.