anyone have any tips to prevent forum spam?

[ZiNgA BuRgA]

Registration security question is generally easy for bots to bypass.  The only thing it protects against is very generic bots.
I've looked at Stop forum spam, but all it does it check username+email+IP against a database, which is questionable in how good that approach is.  For one, usernames are very easy to generate.  Secondly, whether you trust an external service is you call...
Hide sigs and websites doesn't prevent spam - it just tries to make sig link spammers less "ugly" on your board.

I'm working on an experimental plugin, however, it doesn't try to stop all spam, just lessen the impact slightly.  Have you tried Akismet?  Again, it relies on a 3rd party, so it's up to you whether you trust it or not.