The Unregister with Progress command allows you to disassociate a deliverable from a workpack. This command is available on the shortcut menu that appears when you right-click on a deliverable registered for progress. When you use this command, you receive a confirmation dialog. Clicking Yes on that dialog box will disassociate the deliverable with the progress functionality.
The deliverable will remain available to be re-registered, but any progress it had previously accrued is lost.