To report a player in chat just right click on their name and select Report player. You can select the following reporting options; Verbal harassment, hate speech, spamming/advertising or offensive name.
Report a player in a match by either hitting tab to toggle the info screen or wait until the end of the match. Next to each players name there will be a report a player button. Click on the button and select the reason why you are reporting the player. You can add additional text in your report if you like before clicking the submit button.