Mike -- how about adding a 'Message Board Guidelines' link a few places like the welcome header, signup page, and even a little icon in each message at the bottom.

The Guidelines could be a simple popup with the basics:

Use of appropriate content is required. Abusive, profane and vulgar language will not be tolerated.

Personal attacks on other users will not be permitted.

Advertising, solicitations and chain letters are prohibited.

The cutting and pasting of copyrighted information is prohibited.

The posting of e-mail addresses and/or phone numbers of others is prohibited.

Use the "Notify Moderator" button in the message section to report all violators.

Wishcasting and alarmist posts are prohibited.

Any violation of the Guidelines could result in messages being edited and/or removed and the violator being warned or banned.