If wires with non-zero resistivity have been drawn previously and the whole structure must now be considered as a perfect electric conductor, all resistivities can be disabled without modifying the definitions of the wires.
Go to the Setup tabsheet in the main window and select the Settings panel, Fig. 1. If the option Wire Resistivity in this panel is checked, the resistivities are enabled. Uncheck the Wire Resistivity option to disable all of them.