It would be a shame to just 'fix' a 7 speed hub... as it probably weighs 3 or 4 times what a true single speed hub does.

I traded my Sram 7 speed hub for a single speed rear for two reasons. 1: 1st and 2nd gear exhibited so much mechanical friction that climbing in them was irritating and I always had it in at least 3rd gear anyway. So since I was in a middle gear, I might as well just try a single. 2: It saved about 2 lbs of weight in the rear wheel.

You're really cheating yourself if you want to try a single speed and leave that much weight hanging out back. Also: a true single speed rear hub will be MUCH more mechanically efficient - ie, smooth.

DanO