Glyphosate weedkiller, e.g. Roundup, over and over and over and over and over... again. Takes years. Sorry
See Question number 22686

Agreed. And bruise it before spraying too.

Sorry to be unhelpful but a place where I used to work had this problem. The mares tail was professionally treated then the area was covered with heavy duty membrane and concrete. It still broke through and had to be regularly dealt with.

The problem with it is that it has a waxy covering and weedkiller just rolls straight off. I wonder if mixing the glyphosate with washing up liquid would help.....?

Just seen on tv rhododendrons being killed by drilling a hole in the stems and a couple of squirts of roundup poured in. The concentrated is supposed to be the most effective. Might work on the mares tail.

The pros inject Japanese knotweed with Roundup too. You could try mixing the diluted roundup with wallpaper paste to make a gel and paint it on with a brush.