You become a good sport again either with time or by completing missions, or so the game says.
The system, as-is, is still fucking stupid, though.
Yeah. I was getting griefed endlessly last night. Killed, spawn by the killer, repeat. There is no rules in the game saying he can't do that to me. If I had blown up his car just once, I'd have gotten a warning.
Still can't figure out why we can destroy those cars if it's a bad thing.