When Final Cut Pro is rendering media, you can cancel the render at any point in its progress by clicking the Cancel button. If the cancel button seems unresponsive for a moment after you click it, don't worry; the render operation will stop momentarily.
This can occur in some cases when Final Cut Pro is rendering media that requires a lot of processing power (i.e., clips that have a lot of effects or superimposed elements). In such a case, it's only necessary to click the Cancel button once, and Final Cut Pro will cease rendering.
This document will be updated as more information becomes available.