That requires support of the university which the instructor doesn't have, and there are legitimate reasons for senior students to audit junior level courses in

That requires support of the university which the instructor doesn't have, and there are legitimate reasons for senior students to audit junior level courses in a different field from their major. Universities want to encourage intellectual curiosity and cross-field pollination. However, by the time students are senior they likely have filled their non-major electives, so unless they are taking enough classes to get a minor, the class won't count towards their degree and taking it for a grade is all risk and no benefit GPA and graduation wise. That is one of the main reasons that auditing courses is allowed. If the professor did have support, and this phenomenon was really was causing a surge in enrollment, they could make a compromise like having sections that only allowed junior-level same-major students, to consolidate all the pranksters together.