There is no quick way to delete the Mii Creator names but you can override them.
When you go to edit Mii (with any character) and click on the section of Mii Creator it will give you the names and at the top it give you a selection of "Reenter". Since the harddrive can only hold three names at a time you can just enter three creator names you want. By selecting reenter, deleting the existing name, then entering three new names the harddrive will delete previous creators.
Hope I explained that well.