There's a longer Magneto vs. Hulk thread around here somewhere. In it someone had mentioned that Magneto had fought Hulk before years (Decades?) ago was shown to be able to disperse his gamma radiation and turned him back into Banner.
So because gamma is part of the electromagnetic spectrum that Magneto controls... well, there ya go. Being able to take away someone's powers would be a quick knock out. Especially when it's the Mutant Master of Magnetism vs. some 90 bpound noboday scientist.