It's like this Mathewpart, hate casters. Prot pallies are just done in by casers. Our kryptonite.
The advice given is good. I like the los (line of site) pull method myself. Pull them to you. Funny thing about casters is they will run around a corrner to stand in front of you then cast. Making them avial for your Conceration, helping take down multi mob casters. Second that was talked about was Hammer of Justice, which is great for cutting off their spell and keeping them still. But with the 60 sec cooldown (or 45 if you choose to go with Imp Hammer, which most don't) it is short lived, though very effective.
Another option not talked about as of yet is Seal of Justice. Now though it doesn't stop them on command (like Hammer) it does have a fun way of procting when you need it. Yes it's half the time of Hammer at 2 sec stun, but it's great in stoping them mid cast and cancelling the spell.
Another option to add to your spell defence is Spell Warding and Pursuite of Justice, in a
build like this.
Good luck.