Federal troops must enforce chain of custody on all ballots, and destroy any ballots that lack provenance.
As a former precinct captain in Fairfax County, Virginia, chain of custody on all ballots is an absolute priority, and everyone working an election has to sign their name that there was no cheating of any kind. Stray ballots are not destroyed but neither are they counted.
Frankly, we spend most of election day counting over and over again, and making sure our counts all match. We also spend a lot of time signing forms. I honestly can't figure out how anyone could cheat.