@Z0mb13130013ear If you've already sent the ghost to the Netherworlds, there should be an option (once you click on the Urn) to destroy the Urn. If that didn't work, you may want to check out the link/thread below, as it contained a similar problem. :) (the link is slightly spaced, as I am still a new member so it doesn't allow me to post links :P It is just spaced before .com)
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forums.thesims .com/en_US/discussion/827843/solved-is-there-a-way-to-delete-tombstones#latest
Hope I helped! x)