Do you have any guess, why he plunders?
If it's just for the value of the cache container or it's contents, downgrading to real cheap stuff might be a solution.
But if it's a cry for attention, you better do nothing instead of feeding him. Let him steal the rest of the caches in the area, wait a decent time after the menace has stopped and hope he has completely lost interest. Yes, that's hard to do...