Aye, this is an interesting rule, but how do you know someone is sharing an account?
I know that there are some people who have little brothers or whatnot that let them use their account or whatever to play the game, but most people have learned not to let that happened un-attended to by now.
Now, while I understand the thing about sharing admin powers, which I'm fairly certain isn't allowed whatsoever, staying online for "too long" isn't really a rule, so that one won't be of much assistance - in fact, if someones playing fairly on the server for several hours a day, that's a good thing, meaning they like the server.