I am aware of the pending status and am glad to discover you clear that flag when you get the message back. For non subscribed members you will never get the message back and the pending status will stay forever. Why can't you simply delete pending posts automatically after 24 hours? That will achieve the desired result.
No we don't want to stop all users from posting via nabble, only those who are not subscribed to the mailing list.
As I said without this we get email replies to posts that our email subscribers have never seen. Not to mention posts on Nabble from peope who are not subscribed.
Most of the pending posts are honest mistakes made by users who don't know how mailing list works. Some pending posts are because of mailing list server issues, for example, a mailing list server might be down for the time being ... If we just delete the pending posts by an arbitrary rule, then we may overkill and innocent people may never get a chance to fix their problems.
We also don't have a good delete function at this point. If we delete a post, it will be gone for ever. A more reasonable way is to "detach" it from its parent. So, the deleted post is just removed from its forum or parent thread, but it is still the property of the author. We will have this function soon. After that, we will look at your request for delete again.