Have bidding system. Top 3 bidders will be the Mod.
It takes a lot of effort to get in sync with a mod. Most start off being overzealous and it is very hard for a mod not to take things personally when they are attacked. It's human nature to want to retaliate and once a mod does that, he can no longer do the job.
In my case, I keep my eye on the traffic levels rather than the content. As long as the long term traffic trends are positive, it doesn't matter if certain members log in just to abuse me. I treat every member as a positive contribution to the traffic stats.