catalytic converter

i have a 2001 madza mpv minivan that has 4 catalytic converters(at least that's what i was told).2 are clogged up.car runs bad/smells bad/uses gas.my friend took one of the converters off got all the black stuff out of it and put it back on .this only worked for a litlle while.i live in alabama,there is no emissions inspections.can i just take the converters off and how will that it effect the way the car runs? it is drivable now..just stinky and noisy.can i buy used converters? new ones are expensive $300-400 each. thanks, kim