Apple Final Cut Pro 6 - Manual (Page 468)

Chapter 8 Working with Tracks in the Timeline 135
II
To eliminate tracks from the static region, do one of the following:
m To eliminate video tracks from the static region: Drag the upper thumb tab of the static
region down so that it overlaps the lower one, then release the mouse button.
m To eliminate audio tracks from the static region: Drag the lower thumb tab of the static
region up so that it overlaps the upper one, then release the mouse button.
To eliminate audio tracks,
drag this thumb tab up.