I was getting 20-30 bogus registrations a day. Adding the Register Plus Redux plugin, which adds standard fields like first name, last name, website, personal info, did not slow the flood. The bots seemed perfectly at home with those fields and filled them with random urls and texts. But adding the Stop Registration Spam plugin did the trick. This plugin adds one “challenge” type field. You define a question, such as “What animal goes meeow?” and the user has to fill in the answer. So far this has stopped the fake registration bots dead cold. Not a single successful bot registration.
BTW I don’t completely get the point of fake registrations. The site already has strong spam comment controls and I haven’t had a single spam comment get through. All I can think of is that the mass of fake users could all decide to log on at the same instant, swamp the server, and somehow wreak havoc on the site. Otherwise why are they bothering?