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[dmknights]

Spammers tend to 'Spam' anywhere they can!

A Good idea would be to allow them access to only 1 forum & setup an autoreply mod for threads there that states something like "Please reply with your name/D.O.B., Location" etc etc

REAL members will reply, spammers wont

For example:

Everyone can only post a thread in say introductions.
The auto reply mod will reply saying "Welcome To www.myforum.com Please enter your date of birth to gain access" or something like that

Once they reply, then they get promoted to a usergroup that gives them access to the rest of the forum

Failing that, you could always Moderate new members below a certain post count