Hi,
I guess I'm confused about the scenario. I can't think of anything in InfoPath that would lock the XSN so that it cannot be designed.
If the file is published to a SharePoint form library or as a SharePoint type, users typically click Add Document to open the form. That is, they don't have visibility into the XSN (to right click on it, design, etc.).
Do you expect your users to download the XSN, make changes to it, and then republish it? I guess then the answer would be to limit your users permissions so that they can't publish forms.