Disable a letter key or entire keyboard on Macbook?

My letter 'T' key is not responding correctly. It either doesn't respond or gets stuck and is frustrating. I took it in and the Genius Bar thinks it's the sensor and recommends a new macbook. Can't do that right now so I bought an external keyboard but can't figure out how to turn off the built-in keyboard and even when I don't use it - my 't' key will still start typing on its own!

I know I can disable some specific shortcut keys; is there anyway to disable a letter key? or turn off the entire key board? Or was the external keyboard a waste of time?