[setishock]

You honestly think when you come on-line you're going to stay obscure for long. Search engines and spammers have one thing in common. If you make it, they will come...
About your signup methods. Re-captcha has been for some time now defeated. Bots now can read the text and whiz right in. Some have human masters that sign them up then hand the data to their mindless minions. And other spammers.
I use question and answer as do a lot of others here. It works. Catch to using Q and A is making the question or questions. Related to your site is helpful for those with a genuine interest in the subject matter of your forum. For those outside the subject they either move on or have to do some WAG-ing or some searching. Not too easy yet not too hard. And there in lies the rub. You have to give it or them some thought.
The answer to the question at my forum isn't really all that complicated. Anyone that is a serious anime fan will know the answer. But even they have to stop and think a minute.
I get the oddball signups from time to time. I wash the IP's through stopforumspam to see if they are on the list of badguys. If yes, they're gone. If not I sit back and wait to see what they do.

@sheyworth
That's a time consuming pain in the gazootenblatz. Other than what I proposed above there are many other methods other webmasters use and I'm sure some of them will pop in to this thread and speak their peace.