What led you to believe there's a stored XSS then exactly?
If you don't know where it is, then it isn't really possible to fix. At best, you could remove everything and start again, but you may leave the same hole open next time, so that isn't very useful. (There aren't any known XSS issues in the current releases of XenForo and all of our official add-ons; as usual, I couldn't say whether there are issues in third party add-ons.)