If your slot begins in more than 24 hours, you can cancel it yourself in Staffomatic.
Click on the red cross (see below) to automatically un-assign you from the slot.
If your slot begins in less than 24 hours, you will need to trade it with another Stuwie (more info here). If your slot has not been successfully traded you are still responsible for covering the slot.
If you "no-show" your CPS will be negatively impacted. More info on the no-show policy here.