You can block a user by their profile
Open the profile of the person you want to block
Tap the 3-vertical-dot menu on the top right of the profile page
select “Flag User”
select “Block This User”
You can block a user in "Chats"
Click on "Chats" on the top bar and you will be taken to your "Chats" home page.
This "Chats" home page shows all your open, closed and new chat requests received..
To block a member, click on the member photo on the left of screen to open the full chat.
Simply click on the 3 dots to the right of the screen and from the drop-down menu of chat actions that appears, select "Block this user."
Please note, on mobile phones, the block link is found under the "Account" section at the bottom of the profile. Expand this section, and the block function will be revealed.