Biology is important
Any kind of physical punishment by someone who is not the child’s parent is never appropriate. This relationship has no real emotional depth and no future. Teachers, camp counselors, baby sitters, and others who supervise children neither love nor understand a child deeply enough to make even mild physical punishment acceptable. Grandparents should, if possible, avoid any kind of physical punishment because it will undermine the special, safe relationship their grandchildren need from them.
Any form of spanking is a significant withdrawal from the relationship bank account. Its effectiveness depends on having a reserve in that account. Even with parents, overusing spanking can quickly bankrupt the relationship.