I was just reading a similar question last night, can't find the thread at the moment, but from what I remember the simple answer is no. While you can lock getting into your iPad you can not lock/block certain apps, due to Apple restrictions, correct me if I'm wrong, I'll keep looking for that thread. A way around it would be to put your device in Airplane mode, then at least no new mail can come in and they can't "accidentally" buy apps.