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.

Preventing spam is as easy as gatekeeping. We should be bringing it back. Perhaps there should be multiple layers of social media. There’s deeper and deeper level of authenticity as you go deeper into the network

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.

Spam is an issue mainly because there are conspicuous meaty targets to be spammed, not in fragmented environments. And a target is meaty for spammers because that target has gathered, more often unnecessary, critical mass (large scale services, broadcast type news /thought leaders/influencers). Else even a small overhead for sending requests will drive away spammer incentive.

E.g. OS exploits were targeted towards Windows, not so much for so many of those Linux distros.

We can have spam filters for emails without any identity verifications. It is hard, but it should be possible for IM as well, right?