I suggest that you may not be using your herbicides properly or persistently. Obviously you are contaminating the soil in the area!

Boiling water/vinegar/salt applications are not appropriate for your problem. These are obviously plants with tough under ground storage roots and rhizomes.

Mow or cut back the affected area and allow new growth to emerge. Using a combination of a top kill herbicide and RoundUp, spray (lightly) these plants every time you observe new growth emerging. ONE application will not do it. Scythe is one broad spectrum herbicide that comes to mind as a good product to mix with the RoundUp. Remember, you don't need to get any of these products on the soil at all! They work on plants, not soil.

Apply before the new growth develops a thick cuticle. As soon as more new growth emerges from base, spray again. The plants will eventually die from starvation.