Web-Of-Trust
It is based on the social graph, the connection with people through those you follow directly. It can be used to 'weigh' the trustworthiness of people and then build a spam filter.
Since WoT filters can block unknown people outside your social graph, you can mix it with other techniques to whitelist exceptions, for example PoW (proof of work), some form of payment, profile and text analysis, and so on.
All this refers to the clients' side, on the relays' side we can enforce networks filters and other kinds of analysis.

Thank you for using Nostr :)