If your audience is specific to certain countries, you could tighten up the questions a bit.
What is the first name of the founder(s) of Apple Computer?
What is the first name of the crooner Sinatra?
Etc.......
Also, ask yourself how many REAL members you get signing up each day. If it is less than 15 or so, I'd suggest manually approving them. Of course, this entails you being online each day as you can't ask them to wait too long. I do this in less than 5 minutes a day.
If you manually approve, use your fuzzy logic (brain)...look at username, email and then check where the IP is located. I delete any and all from the other side of the world (due to the nature of my forums)...and, although I have deleted thousands of them, I have never received a single complaint.
We have almost no SPAM problem......even with 10,000 plus visitors on a busy day. The key is to get most of them before they have posting privileges.