One of the best ways to combat harassment during a match is to simply ignore the player. This can be accomplished by opening the score screen in game (default Tab key) and toggling the chat icon off by clicking the button beside the player's name you would like to ignore. This will prevent you from seeing chat and VGS from this player for the rest of the match.
In any chat window, you can type '/dnd' to place you in 'Do Not Disturb' mode. This prevents any player from sending you private messages. Use this option if you are being harassed by a player on the opposing team.
The report button allows you to send Customer Support a report about a player's unacceptable behavior in game. This allows you to select one of four reasons (AFK/Leaving, Feeding, Harassment, or Other) as well as provide a short description of the reason for your report. You can find this from the Score tab immediately following a game after the match has ended.
If at any point these features are not enough to deal with a problem player, you can contact our Support Team by filing a Support Ticket