The speaker icon next to their country or name (if non-anon) down in the country list allows you to mute the country for that game only.

In order to mute, or "unmute" in this case, you simply click the speaker icon on the profile page of the player you wish to mute. If another player, or yourself mutes someone and wants to unmute them you can just go to their profile page and click the speaker icon that will now be at the top of their profile when you look at it. If you hover over it a message should pop up asking if you want to unmute said person.
Note: before muting a player the option to mute them (ie the speaker icon) will be at the bottom of their profile.
Also when you mute someone every conversation they have ever sent to you or anyone else disappears from your screen. Others can still read it and it is still physically there, you just cannot see it.

Hope this helps.

Or you can press the icon in game screen for that specific game only like Draug said. What I was talking about applies more to whole scale muting.

Don't listen to that... You are talking about in game. HE is talking about site wide. Different mute systems.

HE did say "country" which is why I knew it was in game he was referring to.

Ya, my fault. I was thinking about muting in general. Just go with what Draug said. It's simple and to the point.

@Draug - you don't say! Thanks very much for solving this minor mystery!

I was playing a Gunboat game once when someone muted me for a Forum issue, without knowing we were allies in the Gunboat game. Awkward. ;-)