If you close your throttle, and set the trim to 0 (minimum), it should not go into the engine via the fill line if using a third fuel line.

What I do is use a two tube system. One from the tank to the carb, and one to the muffler for pressure. I disconnect the muffler tube and connect a fuel overflow tube from my fuel supply, then connect the pump line from the fuel supply to the carb end of the fuel feed line (disconnect it from the carb) and fuel it that way. When the tank is full, it runs back into the fuel supply via the pressure tube.

I've done it this way for years, and, no, it has not "contaminated" my fuel supply. This method is required in many clubs, by the way.. fuel recovery systems.

You don't have to use all three lines provided in the tank system.

But, if you want the three tube method, using a fuel dot on the fuel-up line, and setting the throttle to closed with the trim to zero will eliminate any fuel into the carb.

CGr