https://meta.discourse.org/t/completely-disable-mailing-list-mode/37334/10?tl=en
Completely disable mailing list mode? - #10 by techAPJ - Feature - Discourse Meta
Jan 5, 2016 - Does any one know of a way to easily hide/disable the option for users to enable "mailing list mode"? In my experience, this is almost always enabled by...
mailing listcompletelydisablemode