As I understand it there is no simple way around the "Deleted Post" issue.
When a post that has replies to it is deleted its "node" still needs to be retained in the Nabble record in order to maintain the structure of the forum. The situation used to be worse. The only option was to delete a post with all its replies.
Not an elegant solution, but if it is only a limited group of members who regularly offend in this way, I suppose you could, for a period, encourage your other users to include full quotes of their posts in their responses. That would leave the offenders with no point to deleting their messages - though it might make the forum look a little messy, until you had convinced your offenders that it pays to follow your forum's rules and conventions and that you expect them to consider their posts more carefully before making them.
Another option would be to subscribe via email to your forum. You would then have the original text of their post in your email. With the use of the Administrator's "Edit_all" permission you could then re-instate the post. If you are going to adopt this approach then it would be wise to add a pinned topic to your forum stating that you reserve your right, as administrators of the forum, to do this. The rules for posting in your forum would then be plain for all to see, and you can feel free take action when required.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.