lock default printer

Does anybody know a way via group policy or REG hack to prevent users from changing the default printer? I have 3 printers inslalled, 1 print device. The printers are named black, color & Adobe PDF. Users are able to change the default printer. I want to lock in the color printer as the default. Any ideas?
OS:Windows XP SP2
Dell 3100cn printer connected via USB
Workgroup , No Domain/AD

TIA