https://www.sos.wa.gov/_assets/elections/wa_vbm.pdf
https://www.cnn.com/2020/09/11/politics/barr-secret-vote-fac...
There's no way that any form of remote ballot is going to be as secure as doing it in person. It's just not possible because you will never know the provenance. Like most things that involve the legal system, it's not the law abiding that we're worried about.
At least it is not a problem I know about where I live.
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.
Also, I'm not sure what you mean by adding marks invalidating the ballot. If you fill out a ballot incorrectly, the poll workers will notify you that there are issues with your ballot.