You need to unscrew that whole freewheel. Then you need to buy a BMX freewheel at your LBS. Screw it onto your hub (grease first), then respace and redish your wheel. Judging by your other post, you will need to invest in some tools to do this, or take the wheel to your LBS.
Or for the cheaper version: use 2 chainwhips to unscrew the smallest cog, then take off all the other ones (depending on the freewheel, some are threaded on, some are slipped on with notches). You will end up with an empty freewheel, a bunch of twisted cogs, and a bunch of spacers. If one of the cogs you have is not too twisted and is the size you want, use that; otherwise, find a cheap bike in a dump/garage sale, and perform same operation to get the cog you want. Worst case, your lbs may have a few old cogs lying around. All that's left to do is put enough spacers on the freewheel body to give good chainline, and thread in the cog you want. Bingo, single-speed, no BMX freewheel, no hub re-spacing, no re-dishing.