If a user is bothering you it's possible to block that user. You won't see blocked users in chat, they're unable to challenge you and they can't send you a message.
To block a user simply go to the profile of that user and click on "Block". It's easy to undo that action by clicking on "Unblock".
If you would like to report inappropriate behaviour by another user please contact our customer support. Please include a screenshot of the abusive messages.