Try to find a mild abrasive that is softer than the steel- the rust is softer than the steel it came from so could be removed in some cases by rubbing it hard with a tough bit of wood! This shouldn't mark the sides of the knife but may shift the rust. Failing that super fine paper or green pan scrubbing pads may do the trick..

Memo to self- don't pay extra for S30V steel over 440C.. <img src="/ubbthreads/images/graemlins/wink.gif" alt="" />