Can't delete API key in Cloudant

I am trying to delete/deactivate an API key in Cloudant that I fear has leaked. In the Cloudant interface under permissions, I can see a list of API keys which I can modify (remove from the database or add/remove permissions), except for the one I want to delete/deactivate. For this key, the permissions checkboxes are deactivated and the 'x' to remove it from this DB is not shown. This keys name is the same as the subdomain I use to access cloudant (<some-hash>-bluemix), so I suspect it is special.

I checked if this is a key created through IBM Cloud IAM, but there are not keys listed there.

Does anybody know how to delete/deactivate this key?

You can revoke that key from the service definition in the IBM Cloud interface

Press the Revoke button to revoke the key and confirm in the next dialogue.

This revokes the admin credential that was created when your Cloudant account was first provisioned. Any applications that use it will stop working and you will need to create new API keys to access your databases in the account.

You can create new service credentials from the same IBM Cloud interface (see left-hand menu in screenshot above).