SPAM and website registration

Since we installed the MOLLOM filtering system, thankfully very few unwanted postings have managed to get through into the forum but MOLLOM does not stop the pests who try to do this from creating spurious user accounts on the site and it takes me a few minutes every day to delete them all. We may be able to reduce this but only by making it harder for people to create accounts on the site - for example by requiring that they provide a valid email address and respond to a verification email before an account is actually created. I suspect that the most effective tactic would be to allow only people who have completed the diagnostic questionnaire to create a user account as the spammers are very unlikely to invest the 20 minutes or so that that takes in creating an account which will be both unusable to do what they want (because of MOLLOM) and deleted within 24 hours anyway. This however would mean that relatively casual visitors to the site would be unable to use the forums. If anyone has any thoughts on the matter I would very much like to hear them before we make any changes to site registration. JB