Thanks for sharing these needs, @Alain_Benard! I've renamed the thread to focus on the first idea of expiring Web Users which I don't think we have a thread for. I know this would be valuable for a range of projects.
We've started some work on submission deletion so it should be coming soon! Please note that you can currently delete a whole form which will delete its submissions. You can then recreate a form with the same form definition if you'd like. The reason individual submission deletion is a little more complicated is that we do want to provide the ability to restore submissions that may have been accidentally deleted.
We're actively exploring designs for managing user access within projects. You can see some of that at Restricting web user access to specific forms within a project and Grouping ODK App users. It would be helpful if you could read those threads and either add a 
or vote if they match your needs or some additional details if your situation is a bit different.