Lets say there is some invasive Black Walnut trees growing close to a property line and are an immanent problem for all the "wonderful" Black Walnut reasons. The owner across that property line neglected the area and now they are full-on trees in need of cutting, but they want to keep them. They are young enough to not be poisoning the soil, yet, but are old enough to be developing fruit on the lower branches.

Hypothetically how would one go about killing off these trees discretely without cutting them down and all the good herbicides are banned?