You can delete an entire app from the developer portal, see: https://docs.mendix.com/developerportal/apps-list/#2-3-selecting-actions or https://docs.mendix.com/developerportal/settings/leave-delete-app#3-leaving-deleting-or-deactivating-an-app
Our problem are deleted in Mendix Stduio. How to delete app in mendix studio?