A better method is to rely on questions from current events news. Make it multiple choice, and make it so that 3 wrong answers out of 5 blocks the IP for an hour.
In these days of proxies using an IP blocking approach is pretty much a dead end. Blocking IPs will mean that you'll kill of groups of people using proxies, and they're so easy to fake only the technically dull bad people will be affected.
Without a blocking mechanism it then just comes down to a question of odds.
I also think you'll be surprised at the high false-negative you'll get with real humans getting the questions wrong :-)