The standard G2 tailcap button only operates as a "momentary on", meaning you have to keep it pressed for the light to stay on. Turn the tailcap clockwise for "constant on".
The z48/z49 tailcap has a "clicker" button. It keeps the momentary on feature, but if you want constant on, just press the button harder until it clicks. Click again to turn off. It has a raised rubber rim to help prevent accidental activation. IMHO, the z48/49 is more convenient to use.
Neither the stock G2 nor z48/49 tailcaps have a lockout ability which disables to button entirely.