Concise description:
There should be a warning when you are joining a game with a person whom you have placed on the foe list.
Specifics:
People can not join your games when you have them on foe you should receive a warning that you are joining a game with one of them.
This will improve the following aspects of the site:
This will let you know that you don't wish to join a game with a particular person.
I recently took part in a tournament and didn't realize until the games were under way that one of the people already in the game was on my foe list. I truly did not wish to play this individual any more, consequently I also received a vengeful rating which had nothing to do with the games we just played but was given to me in response to an earlier honest negative rating.