Is there any electrical component on an aircraft which can never be turned off by a pilot?
One example of a system that cannot be switched off by the pilot that is probably more common to all other airframes than just the one airframe I'm indicating.

Whenever electrical power is applied to a Gulfstream G550, whether it is powered by battery, generators, an apu, or some external power source, each engine's FADEC Electronic Engine Control (EEC) computer will be powered and the pilot can not switch them off.

The pilot could only indirectly turns them off by switching off all power sources.

However, when the engines are running, he can't even do that since there is an engine-driven generator dedicated to powering the EECs in the event of a total power loss.

In fact, while the engine is running or even windmilling above 35% rpm, the EEC's dedicated generator is supplying all power to the EEC and there is no switch or circuit protection that can be manipulated by the pilot to interrupt it.