b) if the website field remains the HTML of the registration form (i.e., no change to default miniBB template for registering new users).
This is the proper one, regarding the newest release of miniBB i.e. 3.0.3.
You don't have to remove this form, it should be surrounded by <!--WEBSITE-->...<!--/WEBSITE-->
, the script will do the rest.
Removing the website field completely is your custom modification and that means, regular/useful members will not be able to put it on form even if they would like to promote their resource instead of giving your forum useful replies. I think that's the worst approach, I'm considering this field as very important in terms of qualified content on forum.
Removing this field actually doesn't mean that the spam accounts will lower in creation. But it will cut off those accounts wanting to bump just their Website and nothing else. Actually, there are no strict tools against spam or false accounts at all; may be then there would be only needed a premoderation
on accounts themselves, not just postings; but this feature is currently not available in Premoderation add-on; you may use "Profiles verification" add-on instead (it's free).