I would definitely cut the roots.
Dig a trench a few feed from the property line, as deep as you can find roots and cut the roots. One cut, the roots in your property should eventually die and rot. Or at least won't steal any nutrients or acidify the soil.
That is your property .you can cultivate it any way you want. Not just you own the land, you ow the ground under it for unlimited depth.
Gardeneer
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