You are probably using the hub to delete the Apps? When the App was already published, it will transfered into the Hub Streams on which the delete buttons are disabled. You can only delete an App on the Hub if it is still not published and still in your Work area.
For you to able to delete the Apps is by using the QMC. Open the QMC, click on Start then go to Apps. Select the app that you want to delete then click on delete button on the lower left part of the page. I hope this helps.