To block another user, visit that user's profile and select "Block" from the overflow menu. 

Blocking a user stops that you and the blocked user from viewing each other's content. You will not be able to see each other's profiles, videos, or comments. However, you will still be able to see each other's chat messages during live streams.

Visit the Privacy section of your profile to see the list of users you've blocked. Click unblock on users to unblock them.