These folks will tell you Ivermectin but I am a nature girl....I like Shaklees Basic H soap, like Joel Salatin uses for all his livestock.
Basic H is a nontoxic, biodegradable, all natural soap made from soy proteins. It is not a poison to the worms, it is merely a surfactant, which dissolves the worm's protective layer of oils and allows digestive juices to kill them.
My thinking? If a dewormer is toxic enough to kill an intestinal parasite by poisoning, I don't want it in my sheep's meat and I certainly don't want it in my soils.
Basic H is not advertised as a dewormer, but generations of old farmers have used it successfully for years.
You can also try DE, some folks like it also. It is an irritant to the worms, cutting into their skins and killing them in this way.