pound down
To strike downward (on something) repeatedly and forcefully. A noun or pronoun can be used between "pound" and "down." We need to pound these stakes down with something heavy so the tent doesn't get blown away in the wind. He went around with a sledgehammer, pounding down the bumps and bends in the sheet of metal.
pound something down
to hammer, flatten, or batter something. Please pound that nail down so that no one gets hurt on it. Yes, please pound down that nail!