Horse tail, Equisetum Arvense, is such a nuisance! This weed has been around for millions of years and its roots can extend to more than 1.5m deep into the ground. Its a really tough plant that can re-grow from the smallest piece of root. Whatever you do, please dont rotavate the area or dig it over by hand as you will only spread the plant further still.
Cut back as much as you can at ground level and when the new growth emerges use a glyphosate based herbicide to kill it off; but be warned that you may need to make up to 5 applications before it shows signs of weakening. Try to keep on top of the problem and spray new growth as soon as it appears. Many people suggest that crushing the stems first will improve the results.