The Fediverse relies on users to report anything which breaks the rules, so that instance administrators can take action. (For example, spam, doxxing, hate speech etc.)
You can report a post by clicking on the "..." below it and selecting "Report".
This report goes to your home instance's admin, who can then block the account from that instance.
You can optionally also send the report to the admin of the poster's own instance. If it breaks their own instance's rules too, their admin may delete the poster's account, removing it from the entire fediverse.
Toggle the switch below your report to do this.