To avoid seeing a user again, you can block this user:
To do so:
> Go to their profile page
> Tap on the three dots (...)
> Choose "Block this user".
Blocked users won’t be able to follow you nor see your posts.
Also, you won't see any of their content.
You can unblock users at any time going to:
> Content is at the bottom of the page
> Tape on "Blocked users"