Your options are deadheading it, brushing RoundUp on it, or pulling it up. If you pull it up, it might dislodge other new plants that are not well rooted. Deadheading will be labour intensive, and I think this plant might be rhizomatous, so it would still spread from the roots. Roundup will kill the whole plant, root and all, but you have to be careful not to get it on your other plants, which is why I recommend applying it with a small paintbrush if you decide to go that way.

Or you could just cut it down, and keep cutting it down, and it will eventually give up. April

Hi, twessel - I hope you followed the good advice from ahughes798 above, if you really do want to get rid of it. We find yarrow to be very invasive here in Colorado, but since your growing season is a bit shorter there in the UP, it may not be so bad.

The flowers DO dry down very nicely for indoor bouquets; I grew them when I lived in NW Indiana. You could also keep them under control in containers, but overwintering might be a challenge. Good luck, whichever way you decide to go.