How can I fix stuck open gas valve on Nardi gas hob?

My Nardi gas hob has four burner valves. One valve is stuck in open position. Valve is push and turn type, (pushing down also turns on spark ignition system). How can I fix it? Usual method of pushing down and turning the valve clockwise doesn't work, even after applying medium heavy force.

Is dismantling, cleaning and repair of valve possible, and if yes, how do I do it? If I need to buy a replacement gas valve, then how do I remove old valve and install new one?

Azhar
August 2008