The Mix frame has a constant diameter head tube so if you trim off a few mm's without concern for what it'll do to the frame, just stay out of the welds.
Carefully measure down from the existing edge of the head tube and mark a cut line all the way around. Now take a hack saw and cut. Work your way around slowly keeping above your line. After you seperate off the extension, take a bastard cut file and smooth out your cut. Lastly, take the frame to a shop that can face it.
I cut the bottom of the head tube on my TST titanium frame because the frame was designed for a shorter length fork than the one I wanted to use. It was fairly simple to cut if off but the facing took a while.
All this said, I'd leave the frame alone. The Mix head tube is not "too" high, just run a -17 degree stem and no spacers.