Sign-up via FB, yahoo, google...
Abuse is checked by a simple parameter - when those accounts were created. @irelandscape / @stemq made such log-in last year

Don't worry about the abuse, there are no people willing to join :D
They also have no reason to abuse the system with multiple accounts, because economically it gives them no advantage.

Indeed it's simple:

• those who want to comment once, @guest_Something_Something, public posting key, let them interact with the platform for free (no fee, no rewards, just a normal, boring internet)
• those who want to sign-up can do it with the existing accounts, just like on every other serious platform

Let them join. Once they are hooked, think about the details.
2FA, private keys, how it works, they will learn in a month.
It's wrong to scare them off even before they join