If you have grass in your topsoil, it can be a pain to control. Grass may be pulled up with a rake or hoe, but unless you get all of the grass stolons and rhizomes, the grass will come back from its roots. Systemic herbicides containing glyphosate can be used to kill grass, but the herbicides will also kill any other plant they come in contact with. One of the most effective methods to kill grass without harming surrounding plants or the topsoil is to block the light with cardboard or newspaper.
Set your lawnmower to its lowest setting and mow down the grass. Rake the grass clippings up and bag them for disposal.
Trample grass flat in areas where you cannot use a lawnmower.
Lay cardboard or newspaper in overlapping layers at least 10 sheets thick over your topsoil in areas where the grass grows.
Cover your cardboard or newspaper with 4 inches of woodchip mulch.
Wet down the mulch with a garden hose to keep it from blowing.
Mix the mulch and cardboard or newspaper with your topsoil using a rototiller after 1 year. The mulch will break up easily and will decompose in your soil.