Amjoie - that is the message you get when you get the option. In my case my ghosts took their tombstone with them - so I am wondering - did you quit the game after you did that and then come back later - as it may be the game had to dispense of them - but if they are not gone when you get back and the game said they are gone to the Netherworld - then yes they are safe to delete. You can put them in your sims inventory and have them sell them and make money also. Tombstones with a ghost are in use whether the ghost is in them or out of them, but generally I have been able to move occupied tombstone to my home lot - so that was weird - generally I just bring a sim with me in the daytime and put the grave stone with ghost in it in their inventory. Send the sim home then move the stone into their yard. So I am surprised you couldn't pick it up. I wonder if it was a glitch, or the cheat, or something changed. I have not interacted with ghosts since Showtime came out as i have been busy adding Venues and things to the many worlds I have in my game - as some even needed Pets things as well, so I figured I'd do both things to all of them and be done with it. But I will check tonight and see whats what with the graveyards -if anything. It may have just been the cheat - but yes that is what the game tells you when Sims are technically removed by the game permanently the safe way.
Empty tombstones are just statues to the game - or yard decor and is always safe to delete. or sell.
