with someone riding an atv through your property i would set up some video monitoring equipment. It seems like they didnt take your advice very well. I would contact local authorities. I dont know if it is legal or not, but you could trap them. I know they have the humane traps, but im not sure if they make them big enough for your application. Then you could take them to an animal shelter and hopefully they would go to a family that actually cares about the welfare of thier pets. I would havea real hard time shooting someones pet, I would exhaustallavenues before I went that far. Now if they are aggressive and I or anyone in my family felt threatened I would smoke them in a heart beat.