How to turn on/off S/PDIF Optical Digital Audio Output manually?

My sound card seems to be meesing up. It switches back an forth from "Internal Speakers" or "Headpohones" to "Digital Output". For some reason the optical source keeps getting flipped on and is really annoying. I tried blowing out dust to see if that was causeing the comptuer's confusion, but that didn't work. Until I can make time to go to the apple store, is there anyway for me to turn off that certain connection through terminal commands or something?

Thanks