From your description and my experience, I think Guard only works the way you want it to if you DO NOT order a unit to attack another. If you don't, and your unit gets slammed into, if the nme disengages, your unit will hold its possition. However (and I'm mostly guessing here) if you set a unit to hold, but then order it to attack, it basically disregards the the Guard order and fights like usual.

The only way to make it work, I think, is to issue a Stop order when the nme flees, but then that sort of renders the Guard order pointless, at least in the way you want to use it.