If you wanted to go spelunking around the registry, you might be able to find them, but there's no guarantee that's where the data's stored.
They aren't harming anything, and the amount of space on the disk they take up is so small it may as well not be any at all. It's probably safer to just leave it be unless you come across a program that will edit that list for you.
When you right click on a blank spot of the taskbar select properties then click the customize button...this bring you to what's called "customize notifications". Many items in this section have long been deleted from my computer but are still listed. Is there any way to delete unwanted items in this section?