Will a Wandering Giant Skunk prevent the combat participation of an already committed helper?
Yes, the wandering skunk stops people helping even if they've already agreed.
&quot;Helping&quot; in combat isn't the same as &quot;agreeing to help&quot;. The rules discuss asking for help:

Only one player can help you, adding his combat strength to yours.
If someone successfully helps you kill the monster, discard it, draw Treasures...

&quot;Helping&quot; means everything from agreeing to the resolution.
Giant Skunk says that other players &quot;may not help you&quot;. It doesn't matter if they've already agreed to help or started helping, because the skunk forbids them from helping.

This is similar to a situation with the Gazebo, which says:

No one can help you. You must face the Gazebo alone.

The FAQ has the following:

Q. What if the Gazebo appears as a Wandering Monster after the player already has a helper?

A. The helper has to back off and let the player whose turn it is fight the Gazebo and the other monster(s), alone.

Again, the person was helping you, but is now forbidden from doing so, so they stop.

As a final point, my take on the rule for settling disputes through loud arguments and the owner deciding is that it's a mechanism for convenience. Since the game is a bunch of random events of wildly varying utility and power, it doesn't really matter whether some of the rules are followed or not and it's more important to keep the game running. However, when you're taking the time to figure out the correct rules outside of the game, there usually will be a correct answer.