how can I delete the video tracks from an iDVD project?

I try to use "canc" botton: the video track is deleted on menu, but when I try to insert a new video, iDVD (7.1.2) alert me: "The maximum number of tracks (presentations and movies) is 99. You can not create other tracks."
Then, how can I delete the video tracks (or presentations) really?

Help me, please!