Wielded wood and plastic weapons (and possibly glass) block passive shock attacks, since they are non-conductive. Plastic could block 100% of the time and other materials less so, mainly to give a fringe benefit to a material that’s normally pretty underpowered.

The passive attack only gets blocked if you actually used the weapon to make your attack: if you’re polymorphed into something with a weapon attack and a bite attack, for instance, the bite will still subject you to the passive shock.