You can use Bayesian Inference http://en.wikipedia.org/wiki/Bayesian_inference to determine whether a message is from spammer, in that case account in question will be blocked and no messages will be sent. This system will be also able to learn when new patterns emerge. Blocking ISP is not a very good idea, because you may block regular users wandering around the world and spammers may use proxy servers outside blocked ISP anyway.
Originally Posted by Flyfishermanbob
Not sure that our forum provider can support this, the first approach has just been to google the new members email address , inevitably if its a spammer it will already have been detected by the spam engines .
Yes its reactive , and it would be nice if it were proactive .....but its a question of balance .... at the end of the day, its still less effort to ban than it is to hack , and in all honesty , forums like ours (even though the target is quite small,) will keep getting hit despite prevention. Sad I know