Without asking "why" I will just skip to recommending you pinch the carb line with hemostats when you fill if that is your set-up. Unless the Nexstar has a full cowl that's a bit of overkill.

That, or plug the vent line and fill from the carb line.

Pull the muffler line, too, so you can see when the tank is full or you'll fill the muffler, too.