Mark -- Not sure if this is a suggestion or a bug, but I'd call it a bug.
It revolves around the Hashtag Permissions setting in Message Policies. In the pick-list, there is an entry stating that "Messages from members can only be tagged with existing hashtags, otherwise the message is bounced." But it seems that what actually happens is that the message gets held in moderation. At that point the Moderator can of course reject the message, or replace the offending hashtag with an approved one. If the message is approved without change to the hashtag, the hashtag is added to the hashtag list.
There's also a "Hashtags Required" checkbox here, but if this is checked and a message is sent sans hashtag, the message is not bounced, but held in draft form awaiting further edit by the sending group member.
To this point I been unable to actually create a bounce via misapplication of hashtags. I acknowledge that I have not checked this behavior against every possible combination of moderation/Spam Control settings.
None of the actual behavior seems inappropriate, and it's possible that I may have overlooked something. But if a non-compliant message is going to be held in moderation, it seems that the Hashtag Permissions pick-list should reflect that.
Thanks for your consideration,