At the moment it's impossible to undo an action (moving, deleting, adding, configuring an activity).in a workflow (and in a campaign workflow).
We should have the ability to go back in time in order to avoid cancelling everything just to undo 1 action
as of 9026 there will be some solution regarding your request : How to use the Audit Trail
I hear you, but I believe it's a different feature. I'm talking about a simple "undo (ctrl-z)" button in the workflow toolbar, next to the "start" or "refresh" button