Right these days I am working out the approach, which should help to stop automated registrations. Intermediate miniBB release should follow next week. You already could check the first implementation under your profile.
The approach is quite simple, I was suggesting it earlier
, based on the suggestion from a customer with the similar trouble. The core idea in it, is that ALL spammers want Website field filled in, despite they don't even understand, that it is not clickable for Guests and not indexed by SEs; so we should disable this field on registrations and enable it only for those having more than X useful postings.
FOR YOUR CASE.
Edit setup_options.php -> set
this will disable registrations. Still, if you are logged in as Admin, you can open the registration URL i.e. index.php?action=registernew
(the visible link will be hidden) and create an account like it would be registered by a regular user.
Further is up to you. Create a new topic, or edit some template, to explain where's what.
And... Please, don't forget to check for a new update in the next days.