Liverworts can be killed using any proprietary moss killer. They are difficult to eradicate completely though, especially in areas which are very damp and well fertilised.
This might sound odd but are you sure these are liverworts? Do you have any ferns nearby? the reason I ask is because the first stage of the life cycle of ferns looks just like a liverwort, followed by the fronding adult reproductive stage.
Wouldn't like you to get rid of something you might want