In the lobby just right click their name and select Ignore.
If you do that during a match I think it won't mute them immediately in that match. They will be muted starting the next match.
If you want to mute them in the match immediately type:
/ignore playername
There is a problem where unignoring a user in the lobby does not remove them from the in game ingore list. While in a match, use:
/unignore playername
During a match you can see the list of users you ingore with:
/ignorelist
Note that usernames are case sensitive. Type the first few letters and use TAB to autocomplete.
