You could edit the image file for the gravestones to remove one or more of them, which would cause any graves using that image to not be displayed in the game. Make sure that you use an image editor that preserves transparency, though. And always back up any game images that you plan to edit, so that you can restore them in case there's a problem.
I'm not sure exactly what you want to achieve, though, so that might not do what you're envisioning.
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Barbara
Unicorn
Last Day of Work