I did not know that a "resteem" i.e., sharing someone else's post could not be undone. It makes sense this would simply be clicking the resteem button again, like every other social network. However, Steem is not a social network. It is a publishing platform. Hence, it does not work exactly like a social network. Instead, Steem is almost a hybrid of a social network and publishing platform.
This is a great article by 
https://steemit.com/steemit/@johnsons/no-option-to-delete-a-resteem