hi
marestail is an absolute nitemare (forgive the pun)
one plant can spread for miles underground across the neighbourhood and its very hard to killcompletely, all you can do it control it.
if your going to use weedkiller you first have to take a knife and scratch the surface off the stem, otherwise it is weedkiller resistant as it runs straight off, spray the weedkiller on the part you have scratched.
then everytime you see a stem appear dig down and cut it off, if you keep doing this the plant cant get its food or light and it seems to weaken it.
i still get the odd stem pop up now and again but its nowhere near as bad as it used to be