If you right click a grave to remove it, it will properly reduce the amount of land it takes but it won't reduce the number of burial spaces available to actually fill with dead people.
1. Build a grave. Note the amount of land used up and the amount of burial spaces you have.
2. Remove the grave by right clicking it. Note again how much land and burial spaces there are.
I don't know if this works on other things that give burial spaces because I am not that far into the game.