I actually really like the NextDoor solution. Want an account? Cool, it'll cost you the price of a postcard to your location. When you receive it, you can open your account. More than 10 accounts at your residence? Hrmm... looks like you're up to no good.

I'd be happy to pay the 61¢ per account it currently costs to mail a postcard if it means a massive improvement in anti-bot bs.