Hi Stanley, it means one of three things:
they share an IP address with a known forum spammer.
or
they have an email address used by a known spammer.
or
they are trying to register with a username that is in use with a known spammer.
If you are trying to register a common username then the chance is high it has already been used in the past by a spammer. ie 'Tom' will be blocked but 'Tom_456' would be ok. Tip - try and add a little complexity to your username.
I receive email everyday regarding this, this is the reply:
hi xxxxxx, if you're having problems with the anti-spam, check your:
username, ip, email, in this database and it will
tell you which one is the problem:
http://www.stopforumspam.com/search
our registration is connected to their database.
(most often caused by common usernames which are in use by spammers)
kind regards
Mike