I just clean them with whatever I would clean the stamp with. A baby wipe, water, soap and water, stamp cleaner just whatever is needed with that type of ink.
I mainly use just one block and after all these years mine is not stained. Now I do clean them when I get something on them, I don't want to leave it and get ink on the back of the next stamp. I also clean my stamp right away unless I am going to be using it multiple times in that stamping session.