Increase the number of admins, which will increase the number of deletions.
Have a bot automatically delete files in certain categories that have
been there since X days.
I would expect that most problematic uploads come from new users.
Disable the creation of new users (or their ability to upload) while the
total backlog is >X files (no pun intended). Have a messaage displayed
prominently to put social pressure on the admins ;-)
(disclaimer: I am not an admin on commons)