To block someone in 2or3, go to the person’s profile page by clicking on their profile picture. Then click on the “…” icon in the top right corner. Select “Block user” and confirm that you want to block the person.
If you want to block private messages from a person, enter the private chat you have with that person. Click on the “…” icon in the top right corner. Then select “Block messages” and confirm that you want to block messages from that person.