I have no issue with a 'web-only" option, as long as it does notI don't foresee using it either, but I don't think it would need to compromise other group functionality.
On the other hand, if web-only is easy and maintenance free, I'm okay,Nothing is ever for free, but I think the core of this idea is minimal. When a group selects this option:
1) Alter the New Topic and Reply message poster so that they generate anonymous posts (no-reply instead of member's email in the From field).
2) Alter the inbound processing to reject posting by email.
3) Remove the Reply To = Sender (and Group and Sender) option, Private Reply, and the Email button in the Directory features. And any other place where the UI allows one member to post directly to another.
4) Remove the footer and digest links for Reply to Group and Reply to Sender from outbound messages.
I think that's it.
I don't think it is necessary to muck about with the Subscription settings or anything else in the system.