Something doesn't make sense here. Singlespeed freewheels (or ANY freewheels for that matter) don't use lockrings. They simply thread onto the hub shell in the normal right-hand direction. They get tighter when you pedal, and therefore they are often difficult to remove.

If you truly have a thread-on freewheel, habilis's advice above is WRONG. I think he is giving advice for disassembling the freewheel mechanism itself.

In any case, trying to remove a freewheel from a hub without the tool is generally a bad idea. You'll probably damage it, you might not get it off, and you'll need to buy the tool anyway which will be around ten bucks. If you need it off now maybe give it a try with the hammer and punch. Otherwise just buy the tool.