How to protect a resource from multiple account creation?

Question

DDoS Protection

Good day, the site uses a form with registration by mail, 5 accounts can be linked to one mail
Let's say a person who has nothing to do decided to create a huge number of mails and register from each (using different ip-addresses, nat or vpn, so by ip do not filter the number of created accounts)
How to protect yourself from spam in this case?

Answer 1

This is a profanation of security, which creates even greater problems. It is necessary to protect yourself not from a large number of accounts, but from the real threats that these accounts carry, such as sending links to Trojans, miners, or acts provided for in the Criminal Code.
Therefore, I categorically do not use applications that require user identification via SMS or in any other way allow revealing the user's identity, because. when data is leaked from your application (more precisely, the user database), and this will definitely happen - this data will be used for illegal purposes against the users themselves who provided it...

Answer 2

Browser fingerprint
Capcha
OAUTH (social networks, etc.)
SMS
methods are listed in ascending order of cost for both you and the user.

Answer 3

Require google mail. Or any other mail where a unique phone number is required when registering mail.