We got a ticket on Tuesday because an ATLAS user couldn't get their files back from one disk server which had run out of its 600 (!) ATLAS mapped pool accounts.
I did a bit of a hacky clean up, but actually this is very safe because, unlike a CE, there are no files involved to be mis-inherited by a subsequent user. The only issue would occur at the very moment that a user tried to transfer files.
The clean up removed the oldest mappings, and even the busiest server was down to ~150 mappings and ~450 free slots, so adequate breathing room was gained.
Sam is going to think about this in the storage group concept and write a more general tidier-upper for all VOs.