How to I stop my discharge Valve letting out water? (Boiler, Vokera, Compac?

Hello all,

For the last 48 hours, the discharge valve (atleast I hope it's the discharge valve, it's a small metal pipe outside the building, but seems to be connected to my boiler) has been letting out hot water. It still continues to release the water as I write this. Why is it doing what it's doing and how can I stop it from discharging water??