i had no idea how to kill it

Command-Option-Escape brings up the ``Force Quit'' dialog. You can also usually control-click on the dock icon if the program's not responding and it'll give you the option to kill it, but that may not have been available if the DVD app had taken over the screen. For that matter I can see that C-O-Esc might not have worked, either, but it probably would have.

There's also a Unix-y way to kill processes, but I won't burden you with that, as I don't think that C-O-Esc would fail when the Unix way would work.