Hmmm, yes that is tricky. You could kill that form rule and instead have an Automation fire when Project is saved and the Status is complete and there are still open tasks. The Automation would reset the status back to some other status like "in progress" or perhaps better yet, "reopened due to tasks still being opened", and then also set an email to fire saying "sorry, but your project has been re-opened as it still has incomplete tasks."
Now, the question is who to fire the email to as the [Last Modified by] will be the Automation owner and not the person who tried to close the project.
So the first step in that Automation will need to be to copy the value in the field [Last Modified by] to a field called [Who to Notify of a Status reversal]. Then you have a field to email to.
