There was an Illidan flaming a Nova in my last game, and I'd like to report him for that. Where can I do so?
In the endgame statistics, when you click on statistics you can see all players, when you right click on the specific player the third option counting from below will be the "Report" option there you can select one reason for the report and give a short explanation.
Report will be taken seriusly there are already some banned players for repeatly offending and verbaly atacking other players
Source : Blizzard customer support