Don't you hate those BackStabbers! I hate them too. Well I have a tactic that will help you stop them, first you need to be zerg and your allied friend (the backstabber) has to be Protoss. First build lots of Lurkers, stick about 5 or 6 Lurkers around every pylon in thier base, then the second they start to BS you just have your Lurkers take out all their pylons instantly, it powers down their whole base in seconds. Then just destroy them, a base without power isn't going to do much.

CreepColony's Opinion: If your ally is a backstabber, he'll probably be suspicious about your Lurkers, so use this strategy with caution.