To delete a release for your app:
The above answer is out of date. Splunkbase no longer allows the deletion of releases by developers. If you'd like a release deleted, please send an email to splunkbase-admin@splunk.com from the email address associated with the app's owner account. Please include a link to the app and the name of the release you'd like deleted.
To delete a release for your app:
It doens't work!
@wazuh I've never found a way to delete a release, however you can make the release not visible anymore.
This will prevent anyone from downloading that version of the application...
Splunkbase used to allow release deletion by developers, but that functionality was removed for security reasons in 2017. garethatiag is correct that you can change the release to no longer be visible.