Yarrow can be both a desirable plant and a "weed", depending on where it is growing. Since this plant spreads by both seeds and roots it can be difficult to control and since it likes a wide variety of soils that is not an easy method of control either.
I just keep it mowed and don't get overly concerned about it.

I dig it out. It'll come back, but you can keep it in check with perseverance.