The only way I can see to stop them from deleting topics is to just deny them the permissions to delete posts. I don't allow any of my mods to delete anything due to an issue I had a few years ago when a mod got ticked off, resigned in the middle of the night and then removed all his posts, which created a huge mess in the forums.
We lost two of our most popular forums to the same problem... The mod decided the site wasn't worth his time, and deleted everything in all of his forums (manually)... I was told to "grow up" and "get a life"... The site has never been the same in regards to incoming traffic.
