Persistent Gravestones

So I had an annoying friend join my RLCraft world, and he kept killing himself on purpose. So now I have a bunch of annoying graves spread across half the world and no way to break them. I even tried turning on cheats. I gave him my dragon bone sword to look at and he killed himself with it, now it’s impossible to get it back. I tried /kill on the gravestones, I tried breaking them in creative mode, can someone make a reliable way to get rid of unwanted graves?