This Heroku CLI plugin cleans up your old apps.
In the context of this plugin, a 'temp app' is one named like foo-bar-1234, that you own, with no other collaborators.
$ heroku apps:destroy-temp
finding temp apps... done
1. immense-atoll-41822
2. hidden-savannah-60715
3. guarded-journey-99652
4. floating-atoll-86710
5. obscure-anchorage-63377
Destroy these 5 apps? (y/n): y
destroying apps... done
Installation
$ heroku plugins:install destroy-temp-apps