I sometimes go to karaoke. The location has changed, but the peole running it and the regulars haven't. There's one guy, "Kyle", who always comes. He's a bit mentally slow, but he does his best. Every time he gets up on stage at karaoke, the regulars cheer him on wildly because they're supportive of him, even if he doesn't so as "good" as some of the other performers. I've heard people who weren't regulars downright badmouth him, saying how terrible it was that they let the R-words up on stage. They cheered just as loud when he was done singing, and I was disgusted. But Kyle keeps coming back and singing, and the regulars love him. He even started coming to improv shows (a lot of the regulars are in a group that does them) and whoever's emceeing the show makes sure to get a few ideas from Kyle at every show. They make sure that whoever is badmouthing him or giving him a hard time as drowned out by people who actually do love him so the OP's scenario never happens. All Kyle knows is that he's got the best group of friends ever who love to listen to him sing and think his ideas are really funny.
If the situation was different and there weren't such great people around, I don't know what I'd do. I definitely wouldn't blame the bar for one offense, but I'd make sure I knew the name of that rap group and avoid them. And if any of my friends mentioned them, I'd tell them exactly what I'd seen in the hopes of turning them away from that group.