A cheap way to make an freewheel hub fixed is to weld the sprocket in place around the thread. While you're at it cut off the brake are too. However, I DO NOT RECOMEND THIS. Unless you know a thing or two about welding and can make a strong one.
This also seems like a waste of a perfectly good coaster brake hub. A "fixie" hub can be had for $50 or so, and will get the job done better than a poorly cut and welded coaster brake hub.