Since webmention.io collects received webmentions, there needs to be a way (authenticated API or UI) to persistently delete a received webmention. That is:

• notify webmention.io (via UI or API) that a particular received webmention should be deleted from webmention.io's collection (that is, no longer returned in any API calls etc.)
• if webmention.io receives another webmention with that same 'source', it should reject it (hence the "persistent delete"

This is essential for removing specific abusive webmentions (whether comments, or likes/reposts by abusive accounts).