That's the Related Files bar: a new feature in DW CS4 designed to aid productivity.
It can be disabled via Edit > Preferences > General, uncheck Enable Related Files.
Out of curiosity, why do you want to disable it?
By removing the link to the stylesheet from the parent page, the related file 'tab' will be removed (since it is no longer related). Is that what you mean? Your last post was a bit umm vague and hard to understand.