If you do indeed have a freewheel hub, you can screw on a FIXED COG (not a freewheel). It has the same threads as a freewheel but does not have a ratcheting mechanism. this means that when the wheel moves the pedals also move - no coasting. An English threaded BB lockring can be used to secure.
Warning-a same-direction lockring
must be very, very tight. Even then, it may still release unexpectedly! A fixed-gear hub has opposing threads to pevent this from happening. Should you decide to try this english-lockring idea, I would advise you to use some form of Locktite on the threads, to keep it from releasing when backpedalling.