To block a user, you can use the "Block and Report abuse", which is located in the lower right corner on each profile picture. The "Block and Report abuse" option is also available in "Messages" via the "Options" menu. You will need to specify the reason of blocking.
To block a user, start chatting with him/her. On the right top corner in your dialog there is an icon like three circles. Click on it. Then press "Complain" and confirm your action.
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