Transform.Translate but ignore rotation on one axis

I have a camera, and I have got it moving around when you put the mouse on the edge of the screen. Whilst this works, if you rotate the camera, it translates but uses the rotation of the camera. You can change the rotation of the camera by holding down the middle mouse button. I used Space.World to get around the zoom effect that it was doing, however now it doesn't move relative to the y rotation of the camera. Basically what im asking is that, is there a way for Space.World to not apply on the y axis?