I'm an admin for eGO (a Mature Gaming Community is our slogan) and I'll tell you our system which works great. It's the warn/kick/ban system. Now, to make sure admin abuse doesn't happen, we use text warnings and condumps later as concrete evidence to show admins were just in their kicks. So, one warning, if they pursue wrongful action, kick from the server, and if they return and do more dickitry, they get a ban. Depending on how severe the action is, the ban time is relative to it.
We do have a whole hierarchy and all sorts of organization to it (division leaders, community leaders, event coordinators) so it's easy to stop admin abuse and remove bans and such. Anyways, thought I'd put my 2 cents in.