Misdelivered Mail, Potential Harassment?

Location: Michigan, US

About a year and a half ago I purchased a home and moved in. Ever since we have moved in, we have received some mail for the prior owner. Whenever it is delivered, I would write 'RTS' and toss is back in the mailbox. It never really bothered me until a couple months ago.

At about 9pm one night, the prior owner showed up at the front door unannounced and introduced herself. She said that she had accidentally shipped something to the house and just asked that I call her when it arrives. At that time I told her about other mail that I had been labelling return to sender. Two days later, her daughter came by because they got the notice the package had been delivered and grabbed it directly from our mailbox. She came to the door and told my wife that she was taking the package.

About a month later, two more packages arrived in the mail with the old owner's name on them without any warning. I called USPS this time and asked if there was something different I should do. They had me write 'REFUSED' on the packages and stick them back in the mailbox. I also had talked to the Post Master and they had me fill out some form to prevent delivery with any mail that had the owners or their children's name on it going forward. The next week the old owner showed up later at night, about 9pm again, and left a note on the door asking us to call when the packages arrived. I called her the next day and left her a voicemail telling her the packages had been refused and we didn't have them any more.

She came back again, left another note asking where I took the packages so she could track them down. I called her back, and this time talked to her directly telling her the packages were refused back to USPS, and that once it was out of my mailbox I have no idea where they went. She seemed to understand and at that time I requested she no longer come by the house, since mail and packages are being refused.

Since that interaction we have not received any more mail with the old owners name, however we have had at least 2 handwritten notes in the mailbox asking to return the packages, as well as junk mail with the names crossed out and literal garbage stuffed into the mailbox. I have a camera and saw someone stuffing garbage in from a car, but wouldn't be able to tell if it was her or one of her kids.

My question is at what point does it become harassment? More than anything I am annoyed that this person keeps helping themselves to our mailbox and showing up so long after the house was sold. A month or two I get, but almost 2 years later seems excessive. I do worry that they seem to not be letting it go, and would like to know if there is something I should do just in case this continues.