You will need another spacer.

The top-cap should compress all the spacers, thus setting the bearing tension in the headset. In your case, it's just tightening onto the steerer, doing nothing.

In your case, you'd need a spacer about twice the size of that gap. The spacer or stem should actually extend beyond the top of the steerer about 2-3mm, as shown here...



(http://www.parktool.com/repair/readhowto.asp?id=65)