You could weld the freewheel together to lock it, but this does not resolve the issue that back pedal pressure could unscrew the freehweel off the hub. Welding the freewheel to the hub wouldn't work since most hubs are aluminum and freewheels are steel. Cheepie wal-mart hubs are somtimes steel, but since they are usually pressed metal their stength shouldn't be trusted. It would just be better to run the thing as a single speed, and get the correct hub later.