Of course they can be taught the difference! Our cockers are always on the sofa with us, but at mealtimes (yes, we tend to have TV dinners) they know they can't come up and go off and do their own thing. If we don't want the dogs up we tell/sign them to get off and they go away until we say they can come back up. We're not very good trainers (only really bother training come, sit, off and no) but they understand when they can be on the sofa with us and when not.