Well I talked to MY lawyer (whom I retain for property line and easement issues), and she said that what he technically did could be viewed as an assault and battery (the battery is the unwanted intentional contact with grass clippings).
Do nothing until a surveyer places stakes (with his Mark) on the property line. Take a picture of them in place (because the other party(s) could be expected to pull them out in anger or dispute). They could have a surveyer, too put stakes in the ground, and you are not permitted to pull them out. Be suspicious of a quick survey done with GPS (even differential GPS using a ground station 'extra' locator). A Court can insist on a chains and theodalite survey done from the local point of beginning. Be prepared to have this all backfire, too: i.e. they may be right as far as property line location.
You are not allowed (in Michigan) to divert or create bodies of water that will cause a neighboring parcel to be flooded for a certain period of time in any season.
Be prepared for sour grapes :laughing:! This could cost you a LOT of money in the long run ($10,000 in my case, and I WON). A District Court judge is not interested in hearing any anecdotal evidence or claims of refused conversation. He just looked at the deeds, the surveys, the written findings of the County Drain Commissioner, some non-computer generated pictures and the history of title (going back to 1837 in my case). These commenst apply via Michigan law.
No lawyer wnts to go before a judeg in these cases, because they usually wind up in the Court of Appeals and those folks can use all kinds of past judgements to accept or refute the District findings (and it can make for new law via clarification). I.e. your case is never unique, so they will shoe-horn your dispute into a similar one's outcome.
Good luck. What ever you do, don't call out the Sheriff or State police, just walk away. A lot of these cases are lost because of a hot temper in front of a Deputy. (I wonder if she'll charge me for this ????)
"Good fences make for good neighbors".