[Lea Verou]

Firstly, what's wrong about being a full-time admin? You then have a lot more time to improve your forums and that's good for your members and for the forums. And I don't think using money from the advertisements (or even from donations) for yourself is a bad thing, you have also put a lot of work there, which would cost if you did it for somebody else.
About MU now... A site is a privately owned area, and you should have the right to throw away members that you don't like there, as you would do with your house. A ban isn't always an acceptable solution, you can't ban somebody because you simply don't want him/her in your forums! Also, he might have friends that you would like in your forums, so banning him would result in them leaving as well. So MU could be of use in that situation.
It's not about ethics, its about simple logic.

Disclaimer: I am not a full-time admin (although I would like to be) and I don't have the MU hack (although sometimes I'm tempted to install it but I always say to myself "Try to bare this, and install it on a worse situation").