It's currently only possible to disable, but not delete a Drop-ins app from the App Console. As Drop-ins don't require an approval step, we want you to be able to disable your key but not accidentally (and permanently) delete your key which could potentially break your app and require an update to fix (especially as that can take a long time in some cases.)
If you registered a Drop-in app using a name you want to use for a different type of app though, you can rename the Drop-in app from the App Console, which will free up the original name for use again:
https://www.dropbox.com/developers/apps
