I believe something as simple as moderated user registration along with their automated filtering goes a long way to combat this. The automated filtering can help remove the spam by recognizing forms filled with random bullshit, or detect input that has been copy pasted multiple times from other registrations, and a human moderator can then have a more manageable amount of registrations to go through and manually accept.

This as well as the email confirmation after the acceptance should already be a big obstacle for massive bot farms. It’s still very much possible for all those agencies that do it professionally and massively with state funding, but at least it would be much more time consuming and expensive for them to do so.