If there's really a problem with excessive killing, then simply develop a cool-down system. For instance, if a player registers 5 kills in 5 minutes, he's obviously on some sort of spree. It would register differently if it was 5 kills per hour.
You can develop this system to work by tick, meaning that players are restricted to a certain amount of kills each tick. You can also limit the fire of weapons, and the damage itself. You can also increase weapon prices.
There's plenty of ways you can limit crime sprees, and banning one person for it won't be enough.
