Check "Show Supported Features" in DVD Player's "Help" menu. If "Video Zoom" is supported (I think it requires a graphics system with Core Image support), select "Video Zoom" in the "Window" menu. Click the "On" box, select something other than "Normal" at the top-right, uncheck "Lock Aspect Ratio", and adjust the "Width" and "Height" sliders to squash or unsquash the image.
If you play it with VLC
<http://www.videolan.org>
you can select "Aspect-ratio" from the "Video" menu.
You can fix the DVD itself with myDVDEdit
<http://www.mydvdedit.com/index.php?pg=main>
Copy the DVD to your hard drive.
Select all the files in the VIDEO_TS folder and use Get-Info to set the permission to Read-Write.
Open it with myDVDEdit and fix the aspect ratio.
An example of how to change aspect ratio is:
<http://www.mydvdedit.com/viewtutorial.php?t=27e&pg=forum>
Save the result.
Make a disk image file from the fixed VIDEO_TS folder with DVD Imager
<http://lonestar.utsa.edu/llee/applescript/dvdimager.html>
Burn a new DVD from the disk image file with Disk Utility.
