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.

If you check the HW Design Manual then you'll see that it suggests the unused pins should be left floating. Please refer to Table 4 on p.15 and p.16.

One thing you CAN do when laying out the PCB is to ground all pins via a resistor on each pin.

Then you can fit 0 ohms for a solid ground, 1K or 10K to stop the pin floating (without damage if something drives it, plus you can pull it high during test), or infinity (no resistor) if you must leave it floating.

Final decision can be made for each pin during testing.