Picture this:

Have a canvasser or harvester pick up mail in ballots. They take it to a safe house and open the envelopes. If it’s a vote for the candidate they don’t like, put marks on it so it doesn’t get counted. Place it in a new return envelope.

You’ve just cancelled that person’s vote. If someone checked that their vote was counted they wouldn’t be able to see that it was spoilt.

You could potentially just throw away the ballot too, I doubt most people check that they were actually received. But it does seem slightly more likely to raise eyebrows. I’m sure you could chalk it up to just getting lost at the post office.