You can untick the checkbox: “Generate revisions in the form preview as well” to generate the revisions only by pressing the “Save changes” button, and not the “Preview” one, but you cannot turn it off completely.
First, the “Preview” button saves the form’s structure too. So, you can press the “Preview” button for saving the changes of form’s structure (with the checkbox unticked), and the “Save changes” button only when you want create a milestone (with the corresponding revision). Concerning to the revisions, the plugin creates a frame of 20 revisions per form, passed this number the older revisions are removed.