1. You squelch someone by using dot command .squelch GAMEID. You can also rightclick on player's name on roster, but usually dot command is faster. The player won't be able to communicate with you anymore. You can squelch up to 10 players.
2. It works until either you or the squelched player relogs into arena, or until you .unsquelch him. (Use ".unsquelch all" to totally clear your squelch list )
3. Yes.
4. Yes.