So the user is saving their own copy of the form after they've removed all subforms, then reopening their copy and using it as a new form? Or just making edits to what they've already filled out?
I'm not sure if it's possilbe, but my though would be to have an additional button to 'remove all future instances' (or whatever you want to call it), and have a click event scripted to change the initial count of that subform to 0. I'm no good at scripting, so no idea how you'd write that.