Another converse of How do I programmatically create folders like My Pictures if they were manually deleted? and Why do folders like "My Pictures" come back after I delete them? is How do I prevent folders like My Pictures from being recreated?
Starting in Windows 7, there is a group policy called Disable Known Folders which lets you specify a list of known folders which should be disabled.
If somebody tries to create a known folder programmatically, the call will fail with
Note that this policy only blocks creation of the folder. If the folder already exists, then the policy has no effect. (You're locking the door after the horse has bolted.)