As with most questions about Nostr, there is no single answer. Some relays are more aggressive at blocking spam than others. Some clients have tools for minimizing the amount of spam you see, and others don't so much.

If spam is a major concern for you, I would highly recommend checking out coracle.social

In the settings, you can adjust the minimum web-of-trust score a profile must have before you will see their notes. It eliminates virtually ALL spam from your feed, and makes the Global feed actually usable.

The downside is, you won't see any posts from new users, since they won't have any score yet, but that is an important part of how it blocks spam accounts.