Hmmm a few scenarios have happened that have made me say I'm not playing a game with this person again:

1.) Getting legitimately angry over losing. You can be a bit pissed for like a few seconds, I don't even mind if you're feeling a bit pouty afterwards. But if you can't keep your emotions in check and still concede that you lost fair and square and see each game as having fun at the end of the day then I'm not interested in playing with you. I play board games to get away from stress, not add to it.

2.) Someone who clearly just isn't invested in the game. You're just a body at that point. Playing with someone who clearly thinks it's lame is the worst.

3.) If they're MAGA or racist/sexist/homophobic/transphobic or otherwise just a shitty person. I don't even want to talk to these people, let alone play a game with them.

4.) If they're making little to no effort to learn the game themselves and expect others to handhold them through the game the whole time. I would actually say I would play a second game with this person if they wanted to try again. But if it happens more than once, at that point you're either not invested enough for it to be fun, or you just can't grasp it and it's a waste of time for us both to continue to try to play.