If you're grappling them, you might consider readying your action to hurt them if they start to manifest a spell. They'd have to succeed the Concentration check to "cast" in a grapple, and then another check vs the damage you dealt. Forcing more checks = more chance for them to fail.
Bonus points if you can get another character to stand next to the grapple. You don't threaten any squares, but your allies do, so the 'fester still provokes attacks from them. If they also ready an action to smack the 'fester if he casts, that's 4 different checks (grapple, your readied action, ally's readied action, and ally's AoO)!