Addresses permitted to post is ALL, as you have discovered, but you can set any address you wish to deny sending from to be moderated (and thus discarded if you choose) — yes it's untidy but it is workable as long as you tell your subscribers their limitations! If they know in advance that only certain subscribers can post then they shouldn't object and, if you choose, you could override this at any time.
It really sounds as though you're trying to use the wrong tool, so let me ask for some more details:
- How many 'posters' will there be compared to 'others' (ball-park figures)?
- Of those 'posters', will they be posting announcements or bulletins that intend for no answers from anyone (including themselves)?
If answer to 2 is yes, you're using wrong utility perhaps — you'd be better off using phpList and having multiple administrators who are permitted to create/send messages to subscribers — one-way traffic only.