Navigate to the user's profile, either by searching for them on the "Search" screen (second option on teh navigation menu), tapping on their picture when you see them in the home screen or in comments screen, or searching for them in your own followers / following on your Account screen.

On the user's profile, tap on the 3 vertical points in the bottom right, and select “Block.” Then, an alert will appear to confirm the action, if you confirm, the user will be blocked.