If the proprietary NVIDIA driver has been installed using the .run file downloaded from their site, to uninstall stop the X server, login as root to a text console, go to the directory where you have stored the installer and just do
Code:
./NVIDIA-Linux-x86-180.22.pkg1.run -uninstall
of course the name of the binary executable may differ for you. Mine was just an example showing the correct option to pass as argument.