Answer: Probably the easiest way would be to use an herbicide containing glyphosate according to the label instructions. You could also dig it out by hand, being careful to take the roots.
Since it seems a lot of it was used, you may want to investigate carefully to make sure that other plants will do well there. If, for instance, they are underplanting large established maple trees, it may be very difficult to establish new plants in their place.
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