Hello @denmarkguy, this is @notconvinced on behalf of Natural Medicine.

I too feel your pain. I had a similar incident just last year. I recently graduated from spreading the kitchen left overs directly onto the lawn to starting a pile.

The neighbor uses chemicals to deal with his weeds, rather than weed wack and thought he was helping by spraying the perimeter of my compost pile.

To make a long story short, I created a barrier he's not welcome to cross.

Have you thought about building a drainage ditch to protect against future transgressions?