We don't allow any firing into uncaps unless someone is firing out. If someone is shooting out, and you decide to return fire, you'd better be sure who you are shooting at because if you kill the wrong enemy you are considered to be base raping and will be punished accordingly.
How I handle it is, after a yell warning, to spawn in the uncap and immediately turn my back to the battlefield in a visible spot. If I get killed, I issue a kill command to the offender. A kick will be on the next offense, tban (normally for 1 hour) for the forth. If, after their ban expires, they are dumb enough to do it again after all of that a Pban is given.