.squelch insertplayerid
You won't hear from them anymore unless they re-log. One day I hope HTC implement perma squelch.

This is cut and pasted from the help section.
Squelching and Unsquelching
In Aces High, channels or players may be squelched so that their messages no longer appear in the radio buffer. Use the squelch and unsquelch dot commands for this purpose.
To squelch a player, simply type ".squelch GameID" into the radio text bar using the player's game id. To unsquelch, type in ".unsquelch GameID".
Channel 1 may be squelched with ".squelch 1" and unsquelched with ".unsquelch 1".
Squelching and unsquelching channel 6 toggles the game kill messages on and off. As with channel 1, use ".squelch 6" and ".unsquelch 6" for toggling between squelched and unsquelched.
Entering ".unsquelch all" into the radio text bar will unsquelch all players and channels.