Nostr employs several mechanisms to combat spam and prevent abuse on the network:

## Content-Based Spam Filtering

One effective approach is to analyze the content of messages for highly repetitive commercial or spam content. Services like Nostr.Band cluster messages based on content similarity and start dismissing new messages if a group exceeds a certain threshold (e.g. 100 similar messages per hour), assuming it is spam.[4]

## Proof-of-Work

Some relays may implement proof-of-work requirements, increasing the computational cost for spammers to broadcast messages. This makes spam attacks more resource-intensive.[5]

## Relay Paywalls

Since relays have high autonomy in Nostr, they can set payment conditions to filter spam. Some relays may charge a small fee for users to connect and publish messages, acting as a financial deterrent against spam.[3][5]

## Pubkey Blocking

Relays can block specific public keys associated with spamming activities, preventing those users from publishing on that relay.[4] However, spammers can adapt by generating new keys.

## IP Blocking

While less effective since data comes from relays, some open relays try to block IP addresses associated with spam sources as another line of defense.[4]

The decentralized nature of Nostr, with no single point of control, makes combating spam an ongoing challenge. However, the protocol provides tools and flexibility for relays and clients to implement various anti-spam measures tailored to their needs.[1][3][5]

[1][3][4][5]

Sources
[1] Why we need Nostr https://nostr.how/en/why-nostr
[2] Cutting through the spam on Damus : r/nostr - Reddit https://www.reddit.com/r/nostr/comments/121ytwf/cutting_through_the_spam_on_damus/
[3] Nostr: The censorship-resistant social media protocol explained - OKX https://www.okx.com/learn/nostr
[4] Spam filter for Nostr | Nostr.Band https://spam.nostr.band
[5] Nostr: Censorship-Resistant Protocol Creates New Breakthrough? https://www.binance.com/en/feed/post/201908