First and foremost .... 4 questions, especially if you ask all of them each time, are not enough. I have 20 questions, but only ask 2 for any one user.
If those are your only questions, I can guarantee that the have been catalogued and are in the spammer database, so they can be answered by bots.
Second, banning spammers is basically useless. Delete the user and prevent them from registering I. The future with better/more questions.
Also, I suggest using bad behavior + httpbl