Can we ground all the nets in a printed circuit board?
You will need to check the datasheets for each part with unused pins. In many cases grounding them is a BAD idea, though in many other cases it may be CRUCIAL. And in some cases you SHOULD leave the pins floating. For example, do not connect outputs directly to ground, as this could cause a short. Do not leave inputs floating, unless they have internal pull-ups/pull-downs. The datasheets should tell you what to do with unused pins, but even if they don't you should be able to determine what to do on a pin-by-pin basis by analysis. There is no one-size-fits-all solution for this problem, so you will have to sort through the pins individually to figure out how to connect (or not connect) them.