Enrolment

I'm trying to find out how I can set the system to automatically unenroll a student after they have completed the course, so that they can enroll again in the future. I have sites such that students come back for the same course and credit. I can't seem to find the key to do this. The coursed are paid for, and therefore, the "self-enrolment" option will not work. What I need is for the students to be unenrolled 30 days after course completion, and able to re-enroll at that point.