The problem with that type of question is that it can be googled to get the answer, which is how the bots get in.
You need to ask something that can't be, such as asking them to enter the first name of Luke Skywalker or the first 3 letters of his surname. Something like that.
There is a double activation MOD that might do what you want (I haven't read the full list of features)
http://www.phpbb.com/customise/db/mod/a ... ation_pack