I found this via the fine website of radio kingpin Paul Harris. It's a list someone compiled of performers who've been "banned" from appearing on Saturday Night Live ever again.
I put the word "banned" in quotes because I don't think that's the right word when what's really at work here is that Lorne Michaels says, "Let's not have that person on again." He probably says that — or would, if asked about certain folks — about a lot of performers because they didn't particularly impress him as outstanding or (more often) because their careers have simply cooled. Most guest hosts and musical guests don't do the show more than once. I mean, Louise Lasser is on the list because the episode she hosted was famously a disaster due to some personal problems she was having that week. But even if she'd been ultra-professional and the show had come out fine, she probably would never have been on the show again. For that matter, when was the last time you saw Louise Lasser on anything?
There have been plenty of performers who were great on SNL who stand as much chance of being on again as does Ms. Lasser. They aren't asking George Carlin, Robert Klein or Candace Bergen to host again, either…and won't unless those people suddenly become somewhat younger and the stars of a current hit movie.
More correctly, this is a list of known cases where there was some friction or perceived misbehavior that supersedes any assessment of the performer's future worth as a contributor to the entertainment content of the program. Also notable are the cases where a segment of a show is being withheld from reruns because of what someone did. It's an interesting list but it should be called something like "Performers Who Pissed Off Lorne Michaels."