Ever hear of a Kill Key?
I found one on eBay last night. This is not a destructive entry device. It is a destructive denial-of-entry device. It is a key that can be inserted into a lock but cannot be removed and prevents the lock from operating. To make it even more evil, it is designed so that the bow may be broken off, leaving the shank of the key embedded inside the lock. I have elected not to post a photo of it because it would be way too easy for a mischief-maker to create his own.
The person selling these things claims that they are for landlords and others who need to secure the premises and prevent someone with a key from entering before a locksmith can be summoned to change the locks. Since the lock is now rendered useless, the locksmith will have to destroy the old lock and replace it. To me, that sounds like an expensive way of keeping an old tenant out of what should be a vacant home.
Being neither a locksmith nor a landlord, I have never heard of either this particular device or a legitimate need for it.