mwuhahaha, thats the power of the warlock
seriously, try to use insignia's of the alliance/horde or any other trinkets and such that help you get control of your character back.
i think that the shammy's totem to break fear should also work.
When dealing with a warlock, make sure to stun him first with a rogue/warrior friend and keep the stun up while you kill the lock. Make sure he doesn't have the time to cast death coil, thats instant fear for a few seconds, and good locks need only that small amount of time to either cast the big fear, or nuke you down almost completely.
so recap: be quick and don't let the lock do anything, so make sure to stun/sap/etc to make sure you can nuke him down withouth him casting anything.