Wow! Here in Canada it’s the property owners responsibility to fence their property, and until that is done they have no legal recourse if someone else’s animals graze it... I believe this is also the case in most commonwealth counties (UK & AUS).
We free range our cattle and sometimes sheep on our property, occasionally some neighbors, and our national parkland... if you encounter a herd of livestock on a roadway, they have right of way and you can be fined for interfering with them.We also have the right to kill any animal harassing our livestock, pet or otherwise.
It is different in cities and for domestic pets like dogs, but chickens are clearly classed as livestock, especially a large flock. I find it interesting the different farm and livestock regulations between Canada and the USA.
That said, I think that a simple non confrontational conversation with the owner would be where I would start. Most people are reasonable neighbors. Then I would fence my own property first. Once my property was fenced... winner, winner, chicken dinner! They were damaging my grass that I use for my own chickens.
Our sheep occasionally wander down through other properties to the beach on the west end of our island. If people complain we come get them, and we don’t even try to charge anyone for our complimentary lawn care services... they just got a free mow and fertilizer! Most don’t mind too much.