Re: Members can add tags
Ignoring the debate over whether it’s a feature or a bug, good or bad, you could just remove the capability from the member role. It’s coded in bb-includes/functions.bb-capabilities.php line 170, “edit_tags”. You could delete that line and members would not be able to edit tags any longer. I’m not sure if that means they can’t add tags to their own post or not though.
Also, instead of modifying a core file like that, you could probably use a plugin to remove that capability from the member role. Or you could create a whole new role that does not have that capability. I think there was a plugin related to custom roles: