[Egg]

Stripping the Earth of its magnetic field. How would a big object do this? easy as pie.

All it would have to do is have enough of its own magnetic field to slow/halt/reverse the flow of molten iron in the inner core, and thus halt / slow the electric currents that interact and produce the magnetic field.

Then the earth would be stripped of its own magnetic field and all that solar radiation would just pore in until the inner core started to rotate again as this other object moved out into space.