Not sure that's what the OP means by 'multitasking list'...
If I understand correctly, you mean the MultiWindow list from the taskbar, right? If that's the case, you probably created a custom preset including the note you're referring to and now that it's gone, the shortcut remains. If that's the case, when you hit the 'up' arrow on the button and select "Edit" you should see the icon with the note's name but also a single S-Note app. From there you should be able to delete the shortcut you created and just keep the S-Note one in your active list.