Assuming your side of the story is true, can you downlevel them to a more junior position and reduce their salary? If they won't accept that, then the only reas

Assuming your side of the story is true, can you downlevel them to a more junior position and reduce their salary? If they won't accept that, then the only reasonable option is to let them go. Other developers will notice and become disgruntled having a incompetent co-worker making more than them.