how to turn off my keyboard on my macbook? the "a" button is busted and behaves as if it's constantly being pressed down. I have a wired keyboard ready to use but I can't figure out how to turn off the keyboard here. I tried Control + F1 after enabling "Use all F1, F2, etc keys as standard function keys" but that's not doing anything.

This "a" problem is making it incredibly difficult to code.
I would recommend Karabiner Elements. It's the successor to Karabiner, and works on macOS Sierra. There's an option under the devices menu to disable the build-in keyboard while another keyboard is selected, which should do exactly what you want.