If your forum is well moderated and has a policy of the sigs not being allowed until the 100th post (or something like that), then you shouldn't be dealing with spam too much. There are plenty of big forums that allow dofollow sigs and don't appear to be having a problem.
All my forums have dofollow signature, however most people going after the dofollow list of blogs and forums are those who want to spam, learned it the hard way by making my blog comment links dofollow (which are now nofollow).
No signatures until a certain number of posts might be a good idea... I want to allow dofollow signatures in my forum, but when I tried it for a while I got a lot of people joining and posting 'Hi, welcome ' in the introductions section a few times, then nothing else I wish people wouldn't abuse it when their fellow webmasters offered a little help in the form of do follow comment and sig links.
I get alot of relentless spammers selling nike shoes from china, I moderate all new members but they go ahead and wait a day until I approve them and the spam away. Its really annoying I wonder what software they are using.
@ninjashoes - do you have a list of mail servers of known spammer domains to filter user signups to your forum?? If you need a decent list PM me. I've had my forum up over two years and I have a good filtering list of email domains that if you ban them on signup, you won't have a problem. For example, every forum admin knows to ban signups with an email at *@mail.ru or *@list.ru - in fact, a lot of forums ban *.ru and *.ca explicitly. As a forum admin, you have to be proactive and not protective.