It depends on the university that you're trying to contact. I needed to un-enrol from UNSW, so I had to fill in a form and fax it to them. You're probably best off calling or emailing the uni that you want to un-enrol from.
Today I got a letter from USYD, which just finalises the details of my enrolment. Anyways theres a part that says "application to discontinue enrolment totally".