If you only have a couple of offenders, I do NOT recommend creating a whole new policy to deal with it. Make it clear that (1) paychecks are due on x date and (2) failure to comply will result in disciplinary action. Then discipline offenders, right up to termination. I promise you that firing someone who didn't turn in their time sheet, or turned in an incomplete one, will get just as much if not more attention than docking their PTO.
It's legal, but as you have further defined the problem it is a poor way of managing.

