Couldn’t you just set all the categories to ‘everybody’ can ‘see’ only? That puts the forum into read-only mode.
It would still allow logins, though you could disable new registrations. You could try disabling local logins but that might disable your access as well as everybody else’s.
If the ability for someone to log in and do messages and chats is something you want to avoid, you could also try inactivating everybody’s account, bur allow anonymous access. (It seems like we’re about 90% of the way to a ‘static mode’ with what’s already available.)