While conducting a trail class or a masterclass, I come across many students who join , just for the sake of past time , some are even indecent in their speech, what could be a solution to filter such crowd?

Such a situation is bound to happen and does occur quite often. Probable suggestions that I can offer through my personal experiences are 

- Have a introduction session in the initial 15 mins where you ask each student to introduce themselves and also ask questions as to what they currently do, what is the purpose of joining the session and what they expect out of it. Within that interaction itself you will be able to identify the students who are not really interested.

As regards the indecent language, one of the best solutions I have tried out is that the moment it happens, you pause the class and focus the attention on that student. Instead of telling him about the speech used, utilise the statement that he/she has made and turn it into a learning. e.g ask 'Could you repeat that statement again?' (that automatically draws the attention of the entire class towards him). Either he proudly repeats it as if he has achieved something or he withdraws. In case of the former, get other students involved by asking them 'Do you feel he has used appropriate language in the sentence? Do you think the same statement could be rephrased to sound more polite and grammatically correct?' When the one of the other students provides an appropriate reply, you appreciate it or discuss this a bit further if necessary.  Now get back to the problem student and ask him to repeat the corrected statement again. That way you ended up blunting the effort to disrupt the class and avoided any altercation. 

Lastly avoid getting into any kind of altercation with that student, as he is wanting to draw you into one. If he persists, simply mute him for the entire session.

I feel you can continue to teach and in between just ask students to make payment in some time. It will help keep students in check. Also sarcasm may work if students are mature enough. I generally stop talking when students in my class are noisy till there is complete silence. I just stop mid sentence. This trick never failed me. 😊