Why do forum operator need to clean up the IP list anyway?
It's not used in normal forum operation. It's just an FYI for moderators and admins to see in case someone posted something bad and needed to ban that someone.
Cleaning up the IP is not making any sense, because then the next post will put another IP back in, and after a while then the IP will be full again.
So, if you need to start in a new way ('fresh'?), then just remove all users, or better yet, create and use a new database for your forum.