There is interesting thread at http://forums.atlassian.com/thread.jspa?threadID=11768&tstart=0 where Mike Corn has suggested a feature in confluence by which we can restrict the attachment quota.
Users has tendency to put everything (including big powerpoint presentations, pdfs, executable) as part of wiki attachments. And many times it's seen that those attachments are not removed and they stay there forever; putting lots of work on bakup process. In order to prevent this problem; right now we have lowered the attachment size limit (2MB) and asked user to use file storage for any bigger attachments.
I guess having inbuilt control on space quota (so that users can't add any new data till some unused get removed) will be good way to restrict/remove unwanted stuff lying there forever!