I reckon this comes from same ground that any topics, that aren’t kind of organic, written in that category and published in normal way, don’t be federated.
You can do it manually, though.
Same happens with WordPress plugin or if a topic is published with the timer. Or moved from non-federated category to federated one.
I think that is more lacking a feature than a bug per se. ActivityPub isn’t totally ready yet.
You can remove a message from your sight, or from local timeline, if you are an admin of that instance.
But you can’t ever delete a message that isn’t yours. You must be the OP in the original instance to delete — and even then it is pure luck if all others in the Fediverse will follow that deletion request.
So you have to delete it from your forum and I’m not sure if that deletion will be federated too from Discourse. But you should be able to un-federate in Discourse. Anyway… it isn’t that big question because after few minutes no one sees such posting because of high traffic from Fediverse 