If you just prevent new registrations of authors, you will also need to remove the author role from all current users.
See another approach in the first paragraph here: https://kb.wisc.edu/library/page.php?id=31408
The simplest way to disable public submissions to an OJS journal is through the “Journal Sections” interface. While logged in as a Journal Manager, navigate to “User Home > Journal Manager > Journal Sections”. From there, click the “Edit” link for the section you wish to disable submissions for, and select the checkbox labeled “Items can only be submitted by Editors and Section Editors.” This will have to be done individually for each section.