I had the identical problem. The neighbors were so stupid that even with steel concrete form stakes at the marker pins they continued to mow over on my side. I had the police out there more than once, sometimes it got quite loud. I had to paint the concrete form stakes white, as they ran over them when they were rust colored.
Ended up that we quit speaking, for years, this past year the drunk, doper, live in boyfriend died of complications from alcholism, likely liver failure. The womans son is just as stupid and did the same thing as the live in boyfriend did. I've since moved, and now rent the home on a land contract. If mortgage rates ever get reasonable again, I'll be rid of the albatross.
Get the police involved would be my suggestion, although the only thing that remedied my situation was me, moving away. At least when you call the police there is a report filed, and if it ever ends up in court, you have documentation. The police can also trespass him from your property, and arrest him once he is served if he fails to stay off of your land.
I would also video tape him when he brings the lawnmower on your property, you can likely do it from inside the house, if you don't want to confront him. With the invention of cell phones having a camera to document everything is fairly easy, if it goes to court, you have video to back you up.
This wasn't the only problem I had with these neighbors, they also sold dope from the house for years. The local village cops looked the other way when I reported it. Sometimes you ask yourself, why is it the bad actors seem to get away with outrageous behavior.............
Funny thing is since moving out in the country with 4 acres, and virtually no neighbors, I still have trespassing problems. The creek behind my house is partially on my property, and I have ATV riders constantly driving up the creek during good weather, at all times of the day, and night. I'm working on some solutions for this problem currently. My house is about 100 feet from the creek bed, so we hear the coming before we see them. You should see some of the looks of entitlement I get from ATV riders when I confront them, and tell them they're trespassing on my property. I traded one problem, for another.................
Not sure how the law reads in PA, as I no longer live there, but in Ohio, if you mow the property for 21 years, and you also have to pay the property taxes too. This is the only way property can change hands in exchange for grounds maintenance. As long as the deeded owner still pays the taxes on the land in question, no one else can claim it. You'll have to do some research on PA laws to confirm where you stand.