[skippybosco]

The obvious risk is that users that were originally flagged incorrectly that you manually approved would get caught. If this kind of checking happened it would need to only affect users of a certain user group(s).

My vote would be to tie the "re-check" to an activity that would warrant concern (posting, PMing, etc)..

Scenario being a "Registered" group and a "Promoted Registered" group. Users in registered group are checked against StopForum before being allowed to post. Failures can either be quarantined or prevented.

Set up a promotion schedule (time, # posts, whatever) to move users to Registered so your trusted users don't pay performance penalties.