I'm thinking that you mean the steerer tube stops short of "flush" with the stem so that there is a 1.5 cm gap from the stem top surface and steerer's top surface. Is this correct? My initial response was to just cut the excess off the steerer until I thought about what you were saying...
If so, I don't think I do it. You'd would either have to put the stem back in the position where it was (I'm guessing it's too low to be comfortable to you), or purchase a stem with a narrower clamping surface. Hope this helps.