How to set Nikon D70 flash off w/ Nikon CLS SB-600?

The D70's flash will go off and expose the subject about 5%.

How do I turn set it to not expose at all and just control the SB-600?

While in commander mode, set the built-in flash to "--" and it will not affect exposure.

You may still the built-in flash as a catch light in the subject's eyes, or on reflective material, but there's really no way around that unless you reposition yourself or take care of it in post process.

You can turn it down so that it acts as a commander alone. If that's not enough you'll have to filter out the visible light bit of the flash. Nikon SG-31R is intended for that but a small strip of developed film and a bit of tape also does the trick