How do you tell a bad bagpiper to stop playing?

[Southern Breeze]

It depends.
1.) If he is surronded by people with torches and pitch forks, pick up a stick and join in, if you must.
2.) If he is surrounded by people enjoying themselves, consider yourself an effete snob and move on.
3.) If he is all alone (most likely if he is bad enough to warrant any action on your part) offer some useful pointer to improve him.

Excellent choices!