Want to cleanup unused apps in your Azure Active Directory? Well, that's easy - but how to accomplish that if the Delete option is inactive? See the quick solution here.
So, if you find a scenario as here, where you have an app in your AAD and you cannot delete it...
...the reason is that the app is Multi-tenant. As an Admin, you need to modify the app level from Multi tenant to Single tenant, see also here. Open the app properties, scroll down and find the Multi tenant switch and change that to "No". Then, save the new app properties.
You might need to reload the app properties (the page) to see the Delete button active now.
Now, you can delete the unused app. Easy going - do the cleanup!