Microsoft calls the styles in the ribbon the "Quick Style Gallery." The menu entry you want, if you follow the (good) directions in the previous post, is Remove from Quick Style Gallery.
Alternatively, if you are editing a style or creating a new style, there is an Add to Quick Style List check box in the editing dialogue box. It's checked by default, on the (usually reasonable) assumption that if you are going to the trouble to create or edit a style, you probably plan to use it and will want to access it quickly. Uncheck it.
To add a style to the gallery, find that style in the Styles pane (you may have to switch the List selection from its current setting, which defaults to Recommended, to All Styles) and check that box. That will pin the style to the gallery. If you don't do that, it will come and go based on Word's recommendation algorithm. For example, it will display the top three heading levels or one level past the lowest level your document now uses, whichever goes further down the list, up to its maximum of 9. You can pin the lower levels to the gallery if you always want them to be there.
