How to remove a rusty screw and cap from a toilet

The toilet seat has rusted away the screw holes that connects itself to the iron hinges, making the seat removable while the lid and the hinges stays connected to the toilet bowl. I want to remove the hinges and the toilet seat (+ lid) but the hinges have become so rusty that when attempting to remove the cap beneath, the screw rotates with the cap.

I've tried holding the screw head with slip joint pliers but it's so flat and that it's fitted so tightly making it impossible to fix it in place. I've also tried adding lubricant but it's not working.