They seem to come in waves...usually after a Steam sale...though I honestly suspect that sometimes it's EA dropping agitators in so some folks will abandon the servers and try their other games.
Best bet is to get names and post them (in your case, warnings too) in-game so there's a chat record, then bring them to your management in private forums. As an opposing player, I also open the map and report in public chat where the other team can find the player in their uncap.
I hate baserapers...though if the raped team hasn't been able to secure a flag for a while, or isn't dying enough trying to get out of the uncap, it can be understandable. Not an excuse, just understandable.
Perhaps it should be an automatic 1 hour temporary ban after a first-offense? And if the guy's name comes up again, increased penalties?
With the game getting so old, be careful you're not banning players that haven't "got it" yet...especially after a sale when some actually haven't learned the etiquette. I've found that some cheaters are just ignorant at first blush...and that can be fixed. It's a pretty good in-game community for those that don't frequent forums or join clans...once they get that, some become (gasp!) human! lol