DarkSaint, that's actually a good point. Who's trying to capture the wizard will change how they try it.

Do you want the party to actually get captured, or is this just the strategy that the enemy is going to try to use?

Best way to keep a wizard from casting is to render him unconscious somehow. Flesh to stone->Break Enchantment when you want to let him go is probably the best way. But it doesn't let the player fight back; so maybe just put him in an antimagic field.