If you want to remove an application previously added to your Gallery you can delete it through the Manage Apps tab in your admin dashboard.
The Manage Apps tab holds the directory of all live applications in your Gallery. If you're trying to delete an individual app, you can easily use the search to find the app you're looking for.
Once you find the app, clicking the 
will prompt a confirmation popup. Choose wisely; deleting an application will remove it from your store permanently!