Those RST forks are extremely basic - It's basically just a coil spring on one or both sides (wherever there is a +/- screw cap). To have an actual lockout, your fork needs to have oil damping with an adjustable compression valve. The way the lockout works is it closes the valve through which oil flows - that prevents the shock from moving in the compression direction. Your fork doesn't have oil damping - it just relies on friction in the mechanism to damp it.

The absolute best way to go rigid is a new fork, but you'll spend a lot for a replacement that fits properly.