I had to hide it by going to the forum as an admin, clicking Edit settings (not just Turn Editing On and hiding as there was no Turn Editing On/Off function in this context), and changing the Visible setting to Hide in Common module settings.
No obvious, but it got the job done. Moodle seems to have so many ways to disable/hide things and it's not consistent. Some things can be hidden but not disabled. Some things can be hidden multiple different ways from multiple settings pages. Other things need to be disabled. Still other things can't easily be hidden or disabled and have to be handled in the group permissions. Very annoying.