I've read the docs but there is no obvious way you can actually remove an issue or PR from the repository once it's there, atleast not with the tools provided out of the box.
Yeah. I guess you're right in that case. Removing something once it has been approved previously sounds impossible indeed. I guess approval is a final decision. The only option possible (I guess) would be to hide content. But it would still need to be persisted to not break the chain.