We unfortunately do not support a method to mass-hide lessons from several sections at once, but hope to add such support in future updates.
While only 1 course can be assigned to a section, the course that is assigned to a section is just the first one that students will see when they log in. Students can continue to access any other course at any time by clicking “Course Catalog” at the top of their homepage. Once students finish at least one puzzle in any course, that course will permanently “stick” to their homepage (studio.code.org/home) for easy access going forward. The assigned course is not a constraint declaring what course students can or can’t access.
You as the teacher can change the course you’re viewing progress in in your ‘view progress’ tab by changing the dropdown list on the left side to any other course, to see how your students are doing in courses other than the one assigned. Changing the course assigned will just change what students see first when they log into your section. Hope this helps!