Its not in chat, if thats what you're thinking. Pressing the "Q" button brings up a menu of commands. Then hit "M" to open the utility menu. From here you can report someone, mute voice commands, things like that, by pressing the corresponding key listed in the menu.
Mala, if you are falsely reported multiple times by the same person, bring it up to an admin (Press the "i" key to bring up a chat window that only the admins can see.) If there are no admins on, screencap it and put it on the forum in the ban/unban board.