I was an admin on a vBulletin site with 50,000 active users and had to deal with spammers on a daily basis.

vBulletin provided a way to block IP ranges; you could also search for users under any IP range. I would check ranges of IPs and block them if no "good" users were in the range.

I assume robin could do that, too.

Also we implemented a way to prevent new users from posting anything for 24 hours; and all new posts had to be approved by a moderator until the user reached a certain number of posts.

A new user could request to be taken out of "purgatory" and they were given a message when signing up that all they had to do was PM a moderator and prove they were not bots.

Also we implemented a way of not showing new users in publicly viewable lists of users (such as the one that Pinside has in the side bar)... after 10 posts and about 50 days, they would automatically be moved into the new user group.

Also we eliminated ability of unvetted users of sending PMs (which often contained spam).

This cut down on incidents of spam about 95% (at least stuff that main users had to see).