To report a player who is harassing you or is using abusive language, click on the Bubble icon situated next to your opponent's profile picture during the match. This will turn off the chat completely. You can also tick the option to send the copy of the whole conversation to the admin team.
Players abusing the in-game chat may have their accounts temporarily banned or deleted permanently.
Please note that the mentioned dialogue is available to you when you opponent actually says something.
If you had a bad experience during a game, you can report the problem by right clicking on your screen and select 'report a problem'.