I think a vulnerability in SMF is allowing bots to bypass the registration process completely.
Ouch!
Still, it could be worse, it could be a full pwnage.
Errr... you have backups, right? There's a lot of info in here that isn't on any web page.
Also, there are over 36k members which I suspect as being fake - they have made zero posts,
They may have made posts that were deleted. SMF ought to have a flag for "user posted spam".
I think - given that you don't need to register in order to
read these boards, that it would be justified to write a sticky topic to say something like:
We are trimming the system and deleting all users over a week old who have not made any posts. If you are a real user and wish for your account to remain active, please pop over to <General Discussion -> Hello World!> and say hello!Then, in a week or two, do the tidy. It might make things a little bit faster? I guess the hard part will be, does SMF have a way to do something like that, or will it need to be done manually?

Best wishes,
Rick.