How to disable laptop's LCD and use external monitor as only output?

You can switch off the LCD via Control Center:

1. Open System Settings
2. Go to Displays
3. Make sure the button next to Mirror displays is not clicked
4. Select LCD display, named Laptop
5. Move button below from On to Off

try

gksu nvidia-settings

1) It will open a window. Select "X Server Display Configuration" from the list on the left side.

2) Click on "Detect Displays". It will detect the displays attached to your laptop

3) Now, click on the display corresponds to the laptop and in the Configuration, select the option "Disabled"

4) Click Apply. If you are satisfied with the result, click "Save to X configuration file" to make the changes permanent.