Are you on console or PC? for PS4 open the scoreboard, use arrow keys to hover over their names and press triangle (ult button) to mute. Remember that any ult you have is going to be used because Hirez ofcourse, so do it in spawn or before your ult is ready.

Also it seems bugged most probably after the last update and some people refuse to get muted which is a real pain if they happen to be the toxic cunt you're trying to mute