It's not clear what your problem is. A finishing pass is supposed to remove additional material; if you don't want any material taken off, you shouldn't run one. Similarly, if your stock is .75 and the max final thickness is .75, then you don't have any to take away, right? If the flat area you want to leave untouched is inside the "containment area", you might try editing that region to leave it out.
One of the really nice things about RhinoCAM is the free tech support you get from Mecsoft. Have you tried calling them? The tech there can take over your computer, see what you're doing wrong, and show you how to correct it, right there on your computer with your files. Give it a shot - it's going to work a lot better than asking for help here on the Zone.