good job, and well done! this, of course, will require constant vigilance, not merely one single effort. hopefully, a common protocol can be developed - perhaps a set of maintenance tools for instance admins - to help manage large numbers of inactive and otherwise suspicious accounts, especially making it easier and more straightforward for those instance owners with less experience managing large user databases.
in the meantime, perhaps it would be useful to create more extensive documentation and guides for instance admins on the subject?