Okay, this can still be done.
A DVD is a highly compressed video format. However, tools do exist to uncompress it and get it into an editable file. I've had to do this myself on a couple of occasions when the only thing people had was a DVD.
The software below is what I use. It will create a video file that should load into QuickTime 7 or QuickTime Pro, allowing you to eliminate the audio as described by Old Toad above.
I use it with iMovie 06 (and a few times with iMovie 11) and iDVD 11. It has several modes of converting the data. One mode is called "lossless".
http://www.amazon.com/Miraizon-10121201-Cinematize-2-Mac/dp/B0006AAO96/ref=cm_cr _pr_pb_t
By the way, I am pretty good with movie editing using iMovie HD 06 and sometimes the newer versions of iMovie. Old Toad is a much better expert than I regarding video. He has helped me countless times. However, others here can help you as well. Unfortunately, the iDVD section is not visited as often as it used to be. Once you have a working video file you might try posting in the iMovie or QuickTime areas as these are visited more often by experts.