I had similar problem by the Hungarian Lancia forum with SMF 2.0RC4 engine untill this November. I had too many spam users every day trying to register and I was angry because of the many authorisation emails from the forum. I installed some antispam modules, but these modules also could not filter all the spam users.
Then I tried a new idea: I completely disabled the registration in the admin panel and I created an invitation in JPG (because of the published email address) on the main page of my site. I created a register@.. email address forwarded to my address.
I asked everybody who wants to register to send an email to this address with the required nick and a few words about his/her Lancia. Nowdays I register these new users manually with a random password - that can be easily changed in the profile page. It takes just a minute for me to make the registration, and I have about one or two registration(s) in a week.
Since November I haven't had any new spam users - but it is a bit easier by a Hungarian language page, because the Russian and other manually registering "human robots" cannot understand the text in our invitation JPG.
I also installed a module that hides the forum content from the unregistered users and set up a one year interval: the user who does not log in for a year is automatically deleted. Now I am absolutely satisfied with my forum
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