IPB also supports question and answer field on the registration form, which should help stop most automated registrations. It would not stop users from manually registering, and then posting spam (either manually, or through a program with a supplied valid username/password). You can also set new users to go to one group, whose posts are moderated until x posts (i.e. 5) are reached, and then auto promote members of that group to your normal members group. If you need help with that, feel free to submit a ticket.