I guess it's not possible. Here's an excerpt from the Adobe AS3 Language Reference:
"You cannot remove the Settings menu item from the context menu. The Settings menu item is required in Flash so that users can access the settings that affect privacy and storage on their computers. You also cannot remove the About menu item, which is required so that users can find out what version of Flash Player they are using. "
I guess this is going to have to be good enough.