Assuming you want ID verification, why would you need a blockchain? Your identity is deeply linked to who you are and we have identity documents and trusted entities to provide them. These entities can absolutely act as a third-party to verify who you are. This can happen with several different parameters: whether your identity is provided to the site you are using, whether the site your are using is known to your identity provider, whether identities across sites are identical or only linkable by the trusted party. But in all those examples (that are currently implemented by some countries), blockchain is not a requirement.

Assuming you don't want actual ID verification, the choices are even larger but with different trade-offs.

Phone numbers + phone number country + account age + behavior can be used to build a trust score. It might not be bulletproof but it cuts down spam enough for now.

Imagine a messaging app for example, a 1 month old account with a Nigerian phone number cold DMs an account in Australia. The likelihood of this being spam/abuse is extremely high. Vs a 5 year old account that mostly messages mutual contacts cold DMing an account in their own country.

In many countries, phone numbers are a proxy for ID and are difficult to get without having a local ID. The countries which have not secured their phone number system will be less trusted by spam filters.