One approach to requiring prerequisites is to give a test on the prerequisite content on the first or second day of class, with the test scored quickly, and the results worth 20% of the final course grade. Students who do well on the test know the prerequisites, and those who bomb the test have time to drop the course and enroll in one of the prerequisite courses - or to stubbornly struggle with the higher-level material and earn a B grade. It takes some cooperation among the faculty to pull off, but can be effective.