Use your metal ruler and X-acto knife to cut the long line running parallel to the binding edge. Depending on the thickness of your magazine it will probably take between 10 -15 passes to get all the way through. Just take it nice and easy and you'll be through in no time. Once you've made the long cut you can go ahead and cut the smaller sections out, one at a time, nice and easy.
*The remaining unbound portion of your magazine may now be recycled, used as mailer packaging, or rolled up for use as a toy telescope