Mares tail is a very hard weed to destroy.

The leaves of this plant are waxy so the weedkiller does not adhere to them, you have to bruise them first. Either walk on them or whip them with a thin twig in order to break the stem.
After doing this apply the weedkiller and it will get into the plants system and hopefully kill it for you. Use a Glyphosate based product as this will biodegrade on contact with the soil and not taint it.
You can replant after 4 weeks. If you would rather not pour the weedkiller onto the plant you can sponge it on but still bruise the plant.

It will take more than one application but be persistent and you will win in the end.

Alternatively cut the mares tail down and put the area to grass and mow constantly this will eventually weaken and kill the mares tail.