Question about ghosts…

When you select the tombstone in the ghost's inventory it might say "Return to Netherworld". I think that could do the trick but don't take my word for it. I'm usually wrong. :D

I think any ghost can randomly appear walking in neighborhoods under certain circumstances: Spooky Day, Ghost Night at a bar.

Releasing the ghost to the netherworld will delete the ghost sim completely.

Your ghost is playable in part because the urn/tombstone is on your home lot, so if you carried the urn/tombstone in a sim's personal inventory to a non-residential lot instead, and put it there, your ghost would no longer be playable. I don't offhand know the easiest way to do that though-- whether you'd split the ghost off from the main family in Household Manager first, or what.

Put the playable ghost's urn on another lot, like a park or bar. Then, under the household managment, move the ghost to Unplayed households (the "townies").
Destroying a playable ghost's urn will not remove them from the unplayed households. However, Return to Netherworld will delete them completely. Hope that helps!