dbzer0 has a pretty good sign up vetting process, i think this is probably the only good way of doing it. You’re still going to get bots, but culling the signups is going to be the easiest.
TL;DR just move over to dbzer0 and dont leave the instance :)
Also i think on sites like reddit, a lot of the downvoting is just “mass protest” theory in action, people see a comment with downvotes and then downvote it. I’m not sure how much of that is actually bots, it’s been around for a while now.