Leave the editing mode "cleanly" -- saving, navigating back to the article via the tabs, following a sidebar link to another page, hitting escape, whatever. If you stop editing in a way that we can tell is intentional, you'll be asked if you want to discard your changes, and if you say that you do then it'll all get cleaned up. (We can't tell the difference between "I deliberately closed this tab because I want to get rid of this" and "I accidentally closed the wrong tab and I'll be very upset if my changes are lost", unfortunately...)
If you want it to never autosave your changes, you could disable local session storage for the wikis you use at the browser level. But that might have side-effects outside of VE.
We don't have any preferences that'd control the autosave, and I don't think that we have any current plans to implement something like that.