There is plenty you can do, this is one way of keeping them off your water.
Some time ago I was on a river over in the west of Ireland, four Roma's were fishing not too far away from us, locals were on site within minutes and redesigned their cars for them, I think they started out with a fiesta and ended up driving a mazda , car was wrecked.
I am not telling you to damage cars, but the only way to stop them from killing your local water is to make them suffer Financially, they dont understand when you explain to them the rules of the water, but when it comes to parting with money the