How do I disable the on screen keyboard? I have a USB keyboard attached
Null Keyboard is what you want. I do this same thing with my bluetooth keyboard. It does require you manually switch the input method each time, but keeps the pesky on-screen keyboard from popping up.

Another option I've used is to use the method described here to program one of your keys to a Back button, which you can press to remove the on-screen keyboard each time it pops up.