Sadly there is no way to delete app ids. I don't know why Apple prevents this.
You can delete provisioning profiles though. Go to the Provisioning screen, check the ones you wish to delete, and click on the Remove Selected
button.
Update:
April 7, 2013 - Apple made a major change to the provisioning portal website. You can now delete most app ids. But for some reason, not all app ids can be deleted. In my own case, I have three app ids that can't be deleted. I was able to delete several others though.