You can disconnect the battery or pull the ECU fuse to "clear" the codes. It's possible it'll come right back, or it might not. Sometimes you just get a "flyer" outside of the parameters that'll set off a P0420 every once in a while, sometimes there's a real, constant problem, whether it's a cat that's truly no longer efficient, a bad rear O2 sensor telling the ECU the cat is messed up, or something causing it to run rich all the time. Honestly, without a way to watch the outputs of the front and rear O2 sensors, it's about impossible to pinpoint which one of those it is.
I generally recommend a new (quality, meaning Denso or NTK, IMO) rear O2 sensor. They're rarely over $100, and usually are the cause of a P0420 on a car that's otherwise running right.