Since she poisoned it right after you moved in several years ago, it's odd that she waited so many years to poison it again.
Did she admit poisoning it to anyone the first time? If so, that would go a long way as far as prosecution, and she should be called to task for it. With the kids and animals there, someone poisoning your property could go a lot farther than just simple plant vandalism.
Around here, people can trim anything growing over onto their property, and if a herbicide is used on their property that results in damage to your property, they're responsible for it.
Fencing laws can be sticklish. Sometimes fences built inside the property line actually just end up giving the neighbor that property after a certain amount of time. Solid fences sure can help, but even then I'd be concerned about something being tossed over it by a person that could set poison out around kids.