Rather Novice work if I do say so, however I can't really feel your pain, I use to be hired to do that exact same stuff.
However in knowing how to eliminate entire guilds I can offer this advice, clean yout your members list. You have two problems to figure out.
One-Did he get invited by someone else who is in on it, if so you got a network of traitors to eliminate.
Two-Did he do it alone, use charm and charisma to win over the Guild members and time to aquire his position to perform his task. This is the more risky method and the one I used all the time, it also allows option A to take into effect, people start to think who invited him, how did he get in, are there others. I have seen an army of deciet take down a guild faster then an army of men.
Rock Fist has got your back untill your own your feet.