I think I spent a couple of hours yesterday scouring the Flex docs to figure out how to make a tab on a TabNavigator disabled. And it seems like I'm not the only one. After messing with it for a while, I decided to set the enabled property on the child form to false. Which worked.
I went back to the docs and discovered that it had been there all the time:
If you disable a child of a TabNavigator container by setting its enabled property to false, you also disable the associated tab.
Weird, because I know I searched the TabNavigator help page for the word "disable" like a million times.