I like okmister`s idea. If you offer up the opportunity for members to boot out someone, then you`re looking at needing designated forum moderators just to cover your own back-side. The "super secret password" isn`t much of a deterrant.
BUT, the idea that someone can report you for spam or undesireable content is pretty discouraging to most. Though you will have some that will completely disregard that, as one of our dive-goddesses has found out recently.
If that is a route of choice, then be sure to provide a disclaimer/warning in the membership disclosure that will inform potential members that spamming and undesireable content in any section of the forum or messages will not be tolerated and may result in the user being barred from specific sections or completely from the forum.