This is written up on the blog, and repeated here.
The @trustandsafetyWG team manages our server block list, where entire servers are blocked (or defederated from) because of spam, consistent messages that break CoSocial ToS, or other user safety reasons.
As an individual user, you can also choose to block entire Mastodon server, also known as an instance.
The short version is that in the web interface of a post from an instance you want to block, select the three dots, and from the pop-up menu, select the Block domain example.com, then confirm by clicking the Block entire domain button.
Read the full article with screenshots on Nerds Chalk How to block a specific instance on Mastodon.