you could always zip tie your cassette to your spokes. works a treat and is real awesome without brakes.If you have a true cassette hub, Ie the cassette is held on by a lock ring, which once removed the cassette slides off, then your only viable option for this wheel is a single speed.
If you have a freewheel, ie, the whole thing unscrews then you could put a "suicide hub" setup on there. Ie, remove the freewheel, install track cog, and bottom bracket lock ring.
Personally, i dont have a problem with a suicide setup, as long as you run brakes. Other people probably wont agree...
not. there was a fixie at school made this way, i nearly broke it.