[Wifey]

Have you run an IP check on your board to find a matching IP to that the person is using to log on? have you gone into config.php to put that particular mod's account number in the unalterable/undeletable users group in order to keep them from changing themself to an admin when they get on?

Beyond that, have you considered banning those IP's and the partials as well as changng every single mods password for them and not letting any of them on until it's resolved?

Start looking for the solution to the problem. You paid for the software, don't piss that money away before you do everything you can to fix it

Good luck!