Keep Lemmy small. Make the influence of conversation here uninteresting.
Or … bite the bullet and carry out one-time id checks via a $1 charge. Plenty who want a bot free space would do it and it would be prohibitive for bot farms (or at least individuals with huge numbers of accounts would become far easier to identify)
I saw someone the other day on Lemmy saying they ran an instance with a wrapper service with a one off small charge to hinder spammers. Don’t know how that’s going