That's an original request for sure: why wouldn't a project be complete if all of its tasks are complete? Of course PMs could be tempted to close all the tasks to expedite the project closure - or worse, if some tasks are pending completion.
What you describe here is a governance problem: you want an authority to validate the project completion.
As part of my project governance, I've restricted the project manager from closing its project. That is: the PJM will close all of its tasks, the project will be marked as complete. The Workflow rule will pick it up, display a message to the PJM and mark the project as active. Even if the PJM tries to mark the project as Complete, the same workflow will kick-in. The Governance body only has the ability to mark the project as complete, further to review of the project closure review.
Hope this helps