To block a user, open their profile » tap on the ⃠ icon on the top-right corner and click on “block” in the bottom.
You can also block a user while chatting with them, by tapping on the ⃠ icon on the top-right corner in the chat interface.
If someone has already blocked you, you can block him back: search for their profile name, open it, then tap 'Toggle right to block or left to unblock.'
To see a list of users you've blocked, go to the speech bubble 💬> CONTACTS > slide down a little to reveal the options on top > Blocked.
To unblock a user from this list, open his profile by clicking on his preview picture, tap on the ⃠ icon on the top-right corner and click on “block” in the bottom.