DeG - you could, of course, be quite radical (and quite possibly unpopular) and slowly starve the crap performers of the chance to play - i.e. give the better performers more in any spot. And make it clear why you're doing that.

However, perhaps a more positive way would be to do a tour of the local clubs, seek out the better performers and invite them to perform at yours - shame the poor ones who would then be eased away...

Alternatively, if you're at your wits end, why not just say to someone, "You're shite - piss off!" [I'll get me coat...]. :-) Will