Unfortunately,no you can't clean a cat...you need a new one. There is special mesh material and such inside a cat that help convert exhaust fumes and I guess overtime that material looses it's ability to catalize the fumes into supposedly H20 and CO2...well that's the theory behind a cat, you actually do get COx and some NOx.

I thought that emissions warranties extend above a certain time period, I wonder if you can get Honda to replace it for free?