All you need to do is go to the skills tab (type 'k') and select which profession you wish to discard, i.e. mining. A progress bar comes up in the lower part of this screen with a cancel button at then end of it, along with a brief description about the profession below the bar. Discarding of this profession is as easy as clicking this button.
Hope this helps, it's off of memory... I'm in my office far from my WoW game =(