Revoke one polling subscription

Limited, machine-only release. Revokes one feed. The account must hold a current frozen paid Pro or Ultra period, exactly as creation and polling do. The feed must belong to the authenticated customer and to the exact API key making the request; an unknown id, another account's id, and another key's id are all not found, so the endpoint is not an existence oracle across accounts or credentials. Revoking a feed revokes only that feed: the shared API key, the account's other feeds, and other keys of the account are untouched. The account cap slot is released immediately, and the feed disappears from the live listing. Revocation is idempotent: repeating it succeeds and returns the original revocation time.