Yes, you can! Just remove (or rename) the file named...
D3E3A877-38F7-11D9-90CD-000A958D65FE.tif
You'll find this file buried in /Library -> Application Support -> iDVD -> Themes -> iDVD 5 -> 0016_Full_Frame.theme -> [Control-click and choose Show Package Contents] -> Contents -> Resources -> FullFrameTitle.pox -> Contents -> Material. Easy!
Now iDVD won't show the shape any more, nor will it complain about the missing file. The end result is a completely empty theme to customize as you see fit.

