that's totally different. he's not actively keeping someone else from controlling gravity or attempting to usurp control from someone. you think he could keep graviton from affecting gravity in a city? i'd say not a chance, but who knows i guess. that em scan was even further removed from relevance though. he had the city use an external source to amp the em field that still existed withIN the city. there's nothing in the city to completely eliminate the em field. and even if, for whatever reason, it could, i don't see why that would hinder magneto at all. he doesn't draw from the city's em field, but rather the earth's--and even beyond. like i said, jack would have to be able to completely cut the city off from...the earth itself, essentially. i dunno. maybe he could, but i've never seen him do anything remotely close to that. for example, how could jack prevent magneto from drawing iron from the mantle, perhaps even in the form of a volcano or at least a lava flow that would severely damage the city? we've seen his power operate on a global scale when he manipulated the earth's entire field. even weakened his shields have withstood nukes. i mean, in his battle with ironman he was even able to draw power from other planets including the sun. not sure how jack prevents that from happening....magneto just operates at a depth of power beyond what jack has shown imo.
no one to my knowledge has ever over-ridden magneto's control of the em force, not even proteus. i don't see jack being able to do it either. if he can't i don't see how jack would win this. /shrug
oh, and if we add in geo-force, i really think jack would be eff'd but i don't see geo-force having much success against mags either.