As Frances writes, it is not a matter of 'allowing' but more one of 'stopping'.

Probably the only sure method is to have every message read my a moderator before allowing it to appear on the board. The consequent delay would not be acceptable to most members.

The adminstration does a pretty good job of getting rid of the spam. I would be more concered with the number of members who apparently read obvious spam. These are the kinds of messages that contain malicious viruses, worms etc. Why run the risk?