If you're running a blockchain on a single node, it's exactly like running Git on that single node. You can mutate the data locally, even with something as simple as reverting to a backup. Voila, it never happened.

If you're running Git on multiple nodes, it's exactly like running a blockchain on multiple nodes. You can mutate your own local copy, but that doesn't mutate mine, and the set of commits is functionally identical to the union of commits from every node. You can't delete a commit without deleting it from every clone.