How do I remove a user from a subscription topic?

Summary:

Our production Fusion subscriptions were migrated from the Classic Console to the Oracle Cloud Console on 5/17. That migration included Oracle creating Announcements Subscriptions and adding a couple of dozen subscription topic recipients. One of those recipients has since retired so I tried to delete his subscription in the topic details, but it did nothing. It didn't throw an error. Any help would be greatly appreciated.

Content (please ensure you mask any confidential information):

Version (include the version you are using, if applicable):

Code Snippet (add any code snippets that support your topic, if applicable):