Depends on which feed. Follow feed, your client sends a request with a filter that pulls events for a list of people.

Global is just everyone posting to that relay. Or a list of relays you have added.

Clients deduplicate events from people who post to multiple relays.

Relays blacklist people who spam or break ToS - not heard of many people getting blocked at this level for most relays.

Some clients do client side spam filtering based on reports from people in your friends list.