The quickest and down dirty way of doing this is set this permission for the Registered user group.
The problem is that it's not necessarily "read only" as people can still reply and post. However, those posts will merely go into moderation queue.
You'll need custom development to basically do the same thing, permission wise, as well as make template edits to remove things such as the new thread button, quick reply menu, reply box at the bottom of threads, etc.
You'll need custom development to basically do the same thing, permission wise, as well as make template edits to remove things such as the new thread button, quick reply menu, reply box at the bottom of threads, etc.