Mares tail indeed, and no, it will not go away. This plant has been around since the age of the dinosaurs, they grazed upon it, its roots are found several meters down in mines. There are new weedkillers that claim to kill it, you can try one if you want to use lethal chemicals. You can keep cutting its head off, not even mare tail likes that - or you can learn to live with it as many people do. It will die down in the winter, giving you a time of hope that it has gone, it hasn't, and will pop up again come spring. It is an incredibly successful plant. Keep pulling it up where you can, cut it down as much as possible, dig the ground where it is - the black roots that look like thin liquorice strings are very distinctive, get out every bit as the tiniest bit left in the ground will grow a new plant. Try the new weedkillers if you like, I shall wait with interest to see if there is new growth next year or not where they have been used.