You could have a 'performer's selection' team (you and the bartender, you the manager and the waitress, etc) who gives you the authority to tell a c**p performer that they have one song tonight, and it's the last song - OR that the team has decided that performer needs to go off and rehearse his/her material before she/he can play again (in say 6 weeks). You don't need to involve the 'team' much in those decisions, just let them know you need their occasional concurrance on matters.

I have seen some really terrible performers at open mics. And some who keep coming back week after bloody week. They will eventually drive out the non-performing audience if allowed to conitinue. Youhave to find a way to weed 'em out.

It's a good idea to limit all new comers to a very short set. Even one song - with a time limit (no songs longer then 4 minutes). I saw one lunatic drive out the audience at an open mic when the MC gave everyone 5 songs, and this c**p performer sang 5 ten minute songs; all terrible, all badly played, all badly sung, all with missing words for which we had to wait (because presumably they were important). The songs were all deeply personal. He was 30 years old and still angry at his parents for 4 songs - angry at his girlfriend for the last one. The Cmissed the whole set because he chose that time to go smoke refer in the parking lot, then to cruise back and forth between the kicthen and the bar. He apparently didn;t notice that the absolutely c**p performer took up most of the prime time, drove out the listening audience and the rest of the folks who did follow the rules (5 songs or 15-20 minutes) got pushed off the list or played to a room emptied by this as*hole.

As an MC you gota figure out how to make it work, and how to keep it reasonable.