Tackling Spam can be hard depending on how aggressive the forum admins get. The front end (user sign ups) is one place that can eliminate Spammers. Anti-spam routines can take care of a lot of it but it doesn't always stop it which is why IP address blocks are necessary as there are certain blocks of IP address that are know to be Spam farms. Yes some potential users get caught up in these blocks so it is a trade off on how clean the forums admins wish to keep the system with minimum input from them.
The forum I manage uses these IP blocks and sometimes a potential user will email the admin (me) ask why they are blocked. From there I get their IP, handle, and email address for research and unblocking of a particular user. I will explain that I simply don't put up with spammers and if they start spamming I will kill their account including their IP address. Basically I welcome users but not spammers.