You have a property called random Z rotation (or something like that). Thats what you should change (Its totally counterintuitive, I know)
I got really pissed when I discovered that Stardust has that same hindrance... I had to duplicate the emitter just so I could get particles to spin the other way.
Thats one way to do it too with particular if you want the exactly same rotation value (and not a random one), just CW and CCW. Duplicate the effect with properties, and change the rotation value and the seed number.