What free or low-cost user authentication options can I use?

Question

User identification

So that the user cannot create fake accounts ... for Russia, of course, you can charge the entrance through the State Services, but the countries of the former CIS, Europe, Israel, and in the future any others also get to Central Asia.

Answer 1

Authorization via social networks are fine. For the most part, they always require a phone number when registering. This already makes spam attacks more difficult.

Answer 2

SMS gateway to send a verification code to a mobile phone in an SMS message.
To complicate the process for spammers, you can do this only after validating the mail and before trying to send (POST) any data to the site.