The easiest and quickest way is just to do a SQL call if you want them read only for everyone (which it looks like you do). The following SQL command will get rid of the ability to create topics and replies for all groups, including the admin group, so be sure that's what you want
You can always re-enable the admin group if you want afterward.
set CREATE_TOPICS = 0,
CREATE_REPLIES = 0
You'll then need to purge the cached permissions
delete from ubbt_CACHED_PERMISSIONS