[LinuxBender]

Restricting new accounts to only being able to post in one area of a forum and telling people they have to write a message introducing themselves and manually approving people seems to work reasonably well.

Adding to this some forum software have the concept of ranks and the default rank for a new user can have restrictions such as requiring moderator review for any post until that account meets one or more bits of criteria such as number of approved posts or a moderator manually bumps them to an unrestricted rank. This may be useful as the spammers or bots may be fine posting in a board for new members.