With the same tenacity as the plant itself!!! You can treat it with Glyphosate in the autumn before it dies back or keep hoeing off or hand pulling as you see it. This will eventually weaken it.

I Googled horse tail, because there have been a lot of questions about it, and I haven't a clue on what it is. I know pony tail grass, but not horse tail. Anyway, on the UBC Botanical Garden site, there was the same question and the suprise answer was vinegar. Horticultural vinegar if you can get it (20% acidic), but household (5%) is supposed to work as well. It cann't hurt to try.

That will kill the tops, Wylie, but more will come up fom the roots. If you can keep doing that every three days through two or three growing seasons, that will get rid of it.

shoot the horse and the tail will no longer be a problem lol