What do you do when one or more adults is upset or acting inappropriately?
It's usually easier to prevent these things than it is to fix them.

Set the mood
Make sure everyone's doing alright. Put on some music, get some food and drinks on the table (or a lot of drinks, if you really want to be sure).

Pick the right game
The type of games can also make a big difference. Some games are really harsh on those who fall behind. There's nothing that says 'get grumpy' like sitting at 3-5 VP during a 90 minute game of Settlers. If the skill levels in your game group are miles apart, try to level the playing field by picking games that require less skill, like Yahtzee or Pictionary.
Even when the whole group settles on a complex and long game, I find that it helps to throw short and simple games in between.

Invite the right people
The harshest truth is that some people just don't fit well with the rest of the group. You can't please everyone.