If using the FP discussion web there's not much you can do, as postings are
made straight to the board without any form of moderation.
You could use a bulletin board, that perhaps emails you every post, that you
review, then click a link with in the message (that the script adds to each
post) saying "Accept" or "Decline". It would be a bit of work if your forum
is a busy one, to moderate and keep the flow of messages coming, and getting
rid of the junk.
There are such bulletin boards like
www.phpboard.com,
www.yabb-forum.com
etc.
There's an asp/access one on
www.webwizguide.com
And many, many others. Some probably have features to moderate posts, or
even have some form of "captcha" which requires you put in some code or
other (something a spam-bot can't do) before it will accept your post (or
before it will submit it). An Example of a "captcha" is to type in the code
from an image of jumbled letters/characters.
Or have the forum so members must sign up, then then get an email
confirmation before they can post messages. (Something spam-bots may not
necessarily do) The ones mentioned above have that "sign up" capability.