Invincibility lies in the defence; the possibility of victory in the attack. - Sun Tzu.
The stronger the poison, the stronger the locust... all the captchas, tests and hoops to jump through in the world will not change anything but the average "iq" of a spambot.
Prompt reporting and a zero-click thru policy are our only hope and as soon as they get in they have to be eliminated.
There may be a forum software somewhere that offers an option of only allowing three or five posts with a new membership, with further posting ability dependant on approval by "x" members of the first posts, but I've never seen it.
Other forums (with much more, if you can believe it, obscure topics) I am a member of have a MUCH bigger problem with spam.. mods almost everywhere do a sterling job of spotting and nailing spam before it gets in anyone's way and although reporting can be tedious... it's the only way. McCarthyism wasn't a bad system, just McCarthy was a bad guy
The only "practical" way to keep the place spam free is to make all posts require approval by a mod before being published in the forums... but people get impatient with delays and so on.
Noone has tried to sell me viagra or Hot College Dorm Room Cam XXX Girls so far, so you must be doing something right.