Back in the old days when I led group trips, I never had much of a problem with nasty people. I did have the occasional problem with one or another when they couldn't cut the mustard, so to speak.
One guy signed up for a trip. Then a couple of days later, he asked me if there was still room, because he wanted to bring his ex-girlfriend. I told him that she could probably go, but that I needed to talk to her first. He said that he would vouch for her, that she was experienced, yada yada… (note: never accept that!)
They showed up for the trip. Now, this was a short but rugged trip, off-trail. She was two tons of fun. It turned out that she used to be an experienced backpacker, but that she hadn't done any for ten years for obvious reasons. About eight of us got started. Then about four miles in, she fell and broke her ankle, or at least so she claimed. I am no physician, but I assessed the injury as a bad sprain. That greatly altered the trip for everybody. We ended up carrying her gear, and she walked out with an empty pack. By the time we got back to the road, she was walking almost normally.
–B.G.–