I would try to tell them they are fishing of private property first, from a distance of course.
Then by there reaction to your advice, go from there.
If they tell you to stick it, call the police.
If they leave, let it go and hope they get the point.
If they do comeback then def call the police.
This year I stopped at a 17 acre county park to fish for a bit. Well I parked relitively close to where I was fishing and walked straight to my spot. I didn't happen to see the signs that stated the pond was off-limits for a 2 week period before trout season starts.
Luckily 2 guys walking around the pond politely informed me that the pond was closed to fishing. Of course I felt like a complete idiot because when it comes down to it, it is the license holders responsibility to know where they can and can't fish. I was very greatful they told me because if I would have been caught, I could have lost my fishing license, my car, all my gear that I had with me, and faced a $300.00 fine not to mention a probation period where I couldn't buy a license. It was no joke and I was very thankful that they let me know I was in the wrong.
It was honestly a case of not seeing the signs- and trust me, there were lots of em too. Just not on the way from my car to my fishing spot. There was a sign post that is always in the same spot, but the sign was missing.