I could always "forget" the device, but when reconnecting it would never give me the option of guest sign-in.
I ran the "Zune Media Collection Reset Tool" and it immediately took care of that problem. Now I am a permanent guest on both of my computers.
One caveat to this approach...it will wipe out your "collection" within the Zune software (no, it won't delete the files from your hard drive, but it will remove them from your "collection"). It'll be necessary to drag and drop your files back into the software if you want to rebuild your collection.