How do i stop the engine running at a fast idle

Assuming it is a completely mechanical system with no electrical or electronic control there is no actual fast idle facility unless it is provided by a Waxstat cable device fitted into the cooling system.

I think what you mean is the engine idle speed is too fast rather than it is running at "fast idle" which is a term used for a raised idle speed when an engine is cold and decreases as the engine warms until at normal operating temperature the idle speed should be normal. On an old diesel engine it is the Waxstat device if fitted that provides this facility or a hand operated throttle. Very old cars and trucks had hand throttles for fast idle but later this function was provided by the choke or strangler mechanism. Some luxury cars had an electrical device fitted but the fast idle of modern engines is invariably provided by electronics.

I expect you need to adjust the throttle stop screw of the injection pump. This is at the lever where the throttle linkage connects to the pump and there is generally two stop screws fitted that limit the travel of the lever. One screw limits the maximum travel of the lever and therefore the maximum engine speed and this is generally set during manufacture or remanufacture and sealed to prevent tampering.
The other screw is the idle screw which the lever is pulled against by a spring when at rest.
Before adjustment it is necessary to ensure the throttle linkage is not maladjusted and preventing the lever contacting the stop. Disconnecting the linkage until the idle speed has been adjusted will ensure this and then slacken the stop screw locknut, start the engine and turn the screw in or out as applicable until the desired idle speed is achieved, allow the engine to warm up and make further adjustment if necessary and tighten the locknut and reconnect the throttle linkage. Do not adjust any pump screw except the idle screw.

Good luck!