The toxicly nontoxic group

[Kardwill]

New player joins a group, starts playing the session, with no conversation, no warning, no second chance, the DM stops the player and says "I think you should leave". That action is toxic.

Depends of the infraction. I know there are some things that would make me ask a player to pick up his things and leave. For example a new player that tells me his character rapes or torture another PC? Or does that on an NPC "for the evilz lulz" when in a table he doesn't know? Yeah, never gonna fit at my table, so I can spare everyone the trouble and let him look for a more compatible group.
...Okay, I'm a big coward, so I would probably simply quickly end the session, dicuss it with the other players when he's gone, and "forget" to invite him for the next game, but same difference.

For less horrendous stuff, like punching a prisoner at a family-friendly game, a serious discussion would probably be in order, so that the table can tell the newbie he's out of line, or he "doesn't really fit because we want to play X". Although it's unlikely it will solve the problem if the game expectations are different enough.
But I think that kind of things should be told during session zero. Avoiding clashes, laying down limits and wording expectations are the reason why it's the most important part of any campaign.

And yes, sometimes this kind of explosion comes after a long buildup of "small stuff" where the other players bite their tongues, hoping the newbie would take the hint and behave. Communication is hard, and gamers are quite often pretty bad at direct face-to-face conflicts and arguments


As for the second one : We've all had bad experiences with "This Guy", so we as GMs bring this baggage to the game. When it's our horror stories that shape our expectations, it sounds perfectly reasonable, but when it's another GM's, then it will of course feel arbitrary and stupid.
For example, I'm a guy who mostly plays female characters, but I've met many GMs with a "no crossgender play" rule because some guys, at their high-school table 12 years ago, played all their female PCs as sexoholic-stripper-ninjas. And I find this "you can play an elf sorcerer but not a woman" policy VERY frustrating :/
And on the other hand, I've met enough players who used their "chaotic psycho" character to bully the other players or hijack a game, so I'm now VERY wary when a new, unknown player comes up with an amoral PC. And I imagine that would come up as arbitrary for a new player who simply wanted a "shade of grey" character with a redemption arc.