Your only options are manual removal (digging out the entire plant) or chemical removal (using a systemic herbicide to kill the roots). If you use the latter approach, be aware that typical active ingredients (like glyphosate) are non-selective, so do not get the spray on the foliage or bark of any desirable plants in the area. Glyphosate binds well to soil, so anything that accidentally contacts the soil should not pose a threat to neighboring plants, and it will biodegrade into inert compounds.

Follow all product label directions, and be aware that perennial plants sometimes need a repeat application of herbicide before the plant succumbs. (It will also take several days or more for the chemical to move from foliage to roots, so it may look like nothing is happening for several days after application; give the spray time to work and only remove plants once they're fully dead.)

Miri