In this case, what they're doing is clearly going beyond their lawn and negatively impacting you.
It's weird to suggest that "spraying poison on your neighbors" is deemed acceptable, as long as you're standing on your own property when you do it. If they were standing on their lawn throwing rocks at your apple trees, or shooting a gun at your apples, we wouldn't say they're free to do whatever they like. Heck, we don't even let people play loud music if it disturbs their neighbors.
We really need to update our mental models of harm and violence to account for modern possibilities. We should treat harm from pollution exactly as seriously as we treat harm from projectiles. Dying from cancer from your neighbors incidental pollution is just as bad as dying from a bullet from your neighbors errant gunshot.