Is there a timeout on repetitive chat? In RL you're temporarily blocked from chat after repeating the same phrase three times. Solves the repetition issue.
As for sarcasm, yeah that can be used to harass people but only mildly. I consider that about the least toxic possible form of chat in a competitive video game where people want to engage.
If someone hits the ball into their own goal at least someone from the team that got the goal will say "Nice shot!" sarcastically. Mildly toxic but everyone's done it at least once before and so there's usually someone else chiming in with "No problem." Ultimately works out really well. Sometimes people quit but it's rare and it's usually due to embarrassment than anger.
Edit: just to be clear, I agree with the premise that it's an unsolvable arms race with the community. I just meant to say that RL's implementation was pretty good considering that fact.