WoW, a radical move. It may help get the post quality up and keep spam at bay.
It all depends on what niche the forum wants to be in...
i think the forum is good as of now. i don't see lots of spammers.
if the admin goes for invites only it may let the visitors thru search engines feel unwelcome anymore.
i feel that way when visiting a forum as such. i never comeback nor look for invites since the names of these forums sticks in my mind.
My suggestion would be that each member has to wait 15/20/30 days or so to be able to post a llink or have a sig.
having a post limit will make them spam the forum with 25 or 50 posts.. but a time period limit wont..
How about a thread to attract spammers that is only visible to mods and logged-in members with just a few posts under their belt? This might help trap the spammers that only post once.
The topic could be say "recommended SEO services" or "how to get links" or whatever seems to be a spam magnet.
For sig link spammers, what about just making it so that for example, only 'premium' members can have sig links.
Then to become a premium member you either have to pay for it, be chosen by the admins... or if you're feeling lucky try to win the netbuilders lottery which allows you to do anything on netbuilders, anywhere at anytime. ( )
At least you could make something out of it then, since you guys already have to spend your valuable time dealing with spammers.
With content spam, maybe you can try adding the project Honeypot HTTP:BL, which uses distributed honeypots to track active spammers and all kinds of other malicious traffic. That would probably be more effective at stopping spam than using just ZB Block, which I suggested before. Bad Behavior and the HTTP:BL work extremely well together.
I've been planning to set up a default forum, somewhere hidden, to start building a honeypot IP database, just wait for the spammers to find the hidden forum (by using hidden links on other pages/sites), and collect every spam post's IP address automatically into a db and use that to block spammers from posting their junk on my real forum.
But yeah, best way I can think of is to 'make em pay'..