... well, the short answer is you can't.
That's right. The good folks at Google have decided you are not allowed to delete any applications you launched in AppEngine, or any data you created in the AppEngine's datastores. Sure, there are workarounds that may "work around" the problem, but they aren't solutions.
Amazing that so long after AppEngine's launch you still can't do this. And with a limit of 10 free app IDs per user, you can run out of IDs real fast.
Saidi