Quackgrass is a perennial grass, so existing plants will resume growth in spring and autumn (during cool weather) alongside cool-season turfgrass like tall fescue. They would need to be manually dug out or spot-treated with a systemic (leaf-absorbed) herbicide. Such herbicide options will likely not discriminate between this weed and desirable plants nearby, so you'd need to make sure the spray mist or drips do not contact their foliage.

A new generation of seedling quackgrass can be discouraged in the same manner as with other weed seed: by covering bare soil with 2-3 inches of mulch or a living groundcover, since the goal is to prevent weed seeds from touching soil (where they can root and absorb moisture and nutrients) and to deny them access to light. While some pre-emergent herbicide products might target only grassy weeds like quackgrass (you'd need to look through the labels of any product under consideration to confirm which those are), the use of an herbicide isn't strictly necessary if mulch will work instead. Disturbing the soil as little as possible (by not digging or tilling the area) will also limit how many weed seeds germinate any given year.

Miri