Forum OpenACS Q&A: Response to subscriber model
You wrote: "Does anyone know of an effective way to keep non-subscribers from gaining access to certain publishings?"
While I don't know anything about recent versions of OpenACS (you don't mention what version you want to know about), ACS Classic (Tcl) has had this feature pretty much since the very beginning (well, at least since autumn 1999). There should be a Tcl proc at the top of /home/pvt.tcl that does this (forces you to be registered and logged in). Posting a reply to the bboard requires this too (registered and logged in).
The same thing goes with /shared/community-member.tcl. If you're not logged in, you don't get to see the user's e-mail address. This is one of the fundamental concepts of the ACS and is probably mentioned in Philip's books.