Format the flash. Erasing the config isn't fool-proof because there are times people save a copy of the config into a different filename. There's also VLAN.dat that needed to be erased. It's just simpler to format the flash.
My concern about formatting flash is with deleting the image file for the switch. The original post speaks of "disposing" the switch but I am not sure what that really means. If they intend to scrap the switch then having no image file is fine and formatting flash is the most effective way to remove any sensitive information. But if they have any intention of selling the switch or of passing it on to someone else then having no image file is a problem. Perhaps the original poster can clarify what they intend to do with this switch?