To block someone on Instagram:- Tap their username to go to their profile- Tap the 3 horizontal dots (iPhone/iPad) or the 3 vertical dots (Android) in the top-right- Tap Block
To unblock someone:- Follow the first two steps above- Tap Unblock
People aren't notified when you block them. Learn more about blocking people.
Note: You can block someone using the Instagram app on your phone or tablet, but not on the web.