Pry the step treads off of the stringers if you are dealing with a wooden stairway. After you have removed the treads, pry the stringers away from the retaining walls and pull them up and out of the hole. If the steps are stone or brick, loosen them by prying them out with a crowbar, then lifting them up and out of the hole.
Break up the retaining walls by hitting them with a sledgehammer. If the retaining walls are stone or brick, this shouldn't be too difficult. If they are solid concrete, the job may call for a rented jackhammer. Lift the pieces of the wall up out of the hole, or save yourself a lot of work and disposal costs by leaving them in the hole and filling over them.
Fill in the remaining hole with clean fill. Top it off with 6 inches of topsoil. Plant grass seed or ground cover plants on top of it to stabilize the soil.