I think posts were write once.

The netnews standards allow cancellation and superseding of posts. These were sometimes used by posters to retract or edit their posts. They were often used for spam control and moderation. Cancels and supersedes were occasionally also used for hamhanded attempts at censorship ... by entities clueless enough to think they could get away with it.

Cancels and supersedes are themselves posts, and don't travel through the network any faster than the original post. And as sites can restrict whom they'll accept posts from, they can also restrict whom they'll accept or process cancel or supersede messages from.