I have seen many a democratic initiative ruined by trolls, bot accounts, duplicate accounts, and assholes. The best way to ensure that democracy doesn’t spiral into Haiti is to allow only financial contributors of $5 or more to vote (once the boss man has his contributions system up and running). You want to help build this community? OK, then put your money where your mouth is. To be clear, it should still be one vote per person, whether you donate $5 or $500.
Captcha solving services exist. At worse you’re essentially paying low wage workers to solve captcha for you. There are some AI image processing that can solve some captcha but their accuracy can vary.
In the end it boils down to making the cost as high as possible for spammers and also reducing the benefit of having a spam account by rapidly detecting and removing them.
It’s a hard problem to solve even for companies with massive resources.