There is a column in both the post table and the thread table called 'isusenetpost'. The threads are easy to delete as there is also a 'foumid' column there. But there is no way of knowing which usenet articles from the post table are from the forum that needs cleaning as there is no forumid column.

The cleanest way may be to purge all usenet posts and re-pull the news. You can opt to keep articles that are usenet, but were posted by your users in the control panel if that is a requirement.