From Fallen London Wiki

This template adds banners depending on whether certain Semantic MediaWiki properties are set.

In particular, it displays {{HasGuide}} if there is a {{GuideFor}} template on the respective guide page and this page is in the list passed to {{GuideFor}}. Therefore, if you wish to change whether the {{HasGuide}} appears on a given page, but you cannot find it in the source code of the article, you have to edit the guide page and remove the article from the list.