I was thinking maybe you can do some thing with VBA or Vlisp. I use a VBA program to set all of our paths in the options. One of them is the tool palette path. Maybe you could set that path every time a drawing is opened. This would not prevent them from adding the path in each drawing session but maybe it would make it inconvenient enough they would stop trying.
Here is the few lines of code I use to set the tool palette path:
Code:
Set acadPrefFile = ThisDrawing.Application.Preferences.Files
acadPrefFile.ToolPalettePath = "Z:Land DesktopJRToolPalette;Z:Land DesktopLDTToolPalette;U:Land Desktop 2007ToolPalette"
Some one else wrote the VBA program for our company. I have just been maintaining it over the years so I probably can't help you with any details of VBA.