I’m confused, are you using a multi-speed freewheel hub? Not a track hub? Because if so, you should have enough threading. Also, if you are, it won’t have any reverse threading for a proper lock ring, which I assume explains the use of a bb lock ring? I’m having a hard time imagining why you would t have enough thread for a fixed sprocket, but the ‘rotafix’ method suggested above ^^ is what you should do. Maybe also add some loktite for extra security? I sure hope you weren’t planning on doing skids with that setup. Use a front brake and you should be fine.

some photos of your hub and sprocket would be very helpful in properly addressing the problem.