You currently have about 24,500 members and it's increased by over 100 today - so far.
If they all start sending spam - and the technique seems to be to register, then send a spam message a few weeks later - then it's going to become untenable.
My astro society forum has some very aggressive preventative measures, including things such as excluding large email address ranges, including all of China, and on top of that all registrations have to be approved by a moderator. We have managed to get the number of spam registrations down to about one a day on average and that's easily handled.
From a look at the SMF site there are several options that could be used, such as checking spam lists for known spam email addresses or IP addresses.
I'd use a better question, preferably a set of questions. As it is there seems to be a 50% chance that a spammer can get the right answer.
Is it possible to put a security question in the registration email?
Chris