Pretty easy, I think, but let me know if it's TOO simplified.
1) Create a Completed boolean data element that will be set to true when the workflow is finished (must default to false and must be a promoted property that can be editable in the form library - hit the checkbox when publishing)
2) Create a read-only view and set a form option rule that changes to this view if the Completed boolean field is True.
In SharePoint, the form itself won't be read-only, but no one will be able to do anything in the form, because they always get presented with a read-only view. This is one of several methods I use to manage "rights" within the form itself. One risk here is that someone could feasibly edit the properties of the form and set that checkbox to false, then go change the form. Is this an acceptable risk? It is for me in most cases.