Windows

You can use the tweakui tool from Microsoft to adjust the boot behavior, including function key availability.

You can also use it to hide items in both My Computer and Control Panel.

The link is http://www.microsoft.com/ntworkstation/downloads/PowerToys/Networking/NTTweakUI.asp

I have not used this version myself, but the features are in the Win 95, 98, and XP versions of tweakui.

I believe there is no registry tweak or policy modification that would disable the F8 functionality on boot for Win2K.
The only suggestion I have is to replace NTLDR and Ntdetect.com with Windows NT4 versions (SP6a versions, technically).
Here is why: Technet article Q249000 says that if you have a 2000 & NT4 dual boot, and if you installed NT4 AFTER 2000, that the NT4 install overwrites NTLDR and Ntdetect.com
When this happens, the Advanced boot options (with the bar graph and the "Press F8" message appears) are not visible.
So, perhaps if you replace your 2000 versions of these to files (yes, they are important files, so hopefully this won't screw things up!) with the NT4 versions, the advanced startup options will not be displayed.
If in trying this your 2000 box cannot boot up, you can always boot from the cd, run Recovery Console, and replace the files with the original 2000 versions from the cd.

Give it a shot on a test machine first. Good luck!