Nice tip! I'm also a proponent of using your friend the preprocessor to stop you from making big mistakes, along with your best friend, the compiler.
You can combine both using this alternative code:
private bool ValidateUserDetails(string userName, string password)
#warning Not Implemented
with the option "treat warnings as errors" in Release mode.
That has the big advantage of still showing something in the build results when building in Debug, thus inciting your team to go fix the issue before the time comes to make Release builds.