If you see a post that breaks the rules on your instance (such as spam, hate speech, doxxing etc), you can report it by clicking "..." below the message. This message goes to your own instance admin, who can block the account.
Optionally, you can also anonymously report it to the instance of the account that posted it. Just click on the toggle switch to do this.
This means the rule breaker's own instance admin can potentially close the account completely.