There are kits you can buy and this is probably the best course.
The kit consists of the block off plate, a few bolts, and a few nipples to block of the oil port to the carburetor.
Basically you remove the pump, remove the studs holding the pump on, remove the hoses and the oil tank, and install the block off plate and nipples.
I would strongly suggest you drain the unmixed fuel from your ski, and put pre-mix in, and run it a bit to get pre-mixed fuel through the fuel lines.
Most importantly, always remember going forward to pre-mix your fuel - because one mistake of pure gasoline and you have bought the engine.
Next I would respectfully suggest that when you mix your fuel and oil, settle on a fuel / oil regimen and stay with it - buy the best fuel - get the same oil, and mix the same ratio - forever more...
Best wishes - Prof