How do you mute another player?
They must have changed it again, here's the new answer.
- Bring up scoreboard
- Press the right mouse button to enable the mouse cursor
- Left click the player you wish to mute in the scoreboard
- Select "Block Communication"
Hold down TAB to bring up the player list. Then use your arrow keys to go up or down the list to the player you want to mute. Hit the ENTER key to bring up a dialog for the selected player. You can mute him, commend him, or report him.
As far as telling which player is muted, their player icon on the list should have a little muted symbol. As seen in this picture.
It seems as though they have changed the way you mute players. For me, hitting DELETE did not do it, but if I hit ENTER after using TAB and the arrow keys, then I got a small dialog for that player. In it there was a check-box for "Disable Communication from This Player" which muted them perfectly.