I was printing on some fabric the other night. I made a little mess on my pad. The stain is from Riso's white cloth ink.
With a little rubbing and a wet paper towel, the ink came right off. Aah, Bounty saved the day. Now if this was the paper ink (oil), I'm not sure how easy it would have been. Although, I have stained it before with the paper ink. It eventually came off with use (the lint roller brush helped towards the end).
UPDATE: I've had lots of success removing the paper (oil) ink from pad. I use a paper towel with a little soap and water. Comes right off. Comes off easier when the ink is dry.