Before being able to delete a collection, delete all bindings of purple elements in the Designer
– in the viewport: template page elements, and Collection Lists elements in static pages: all link settings must be turned to none
– in the Settings tab: elements can be bound to the Collection by a background image, a background color, a text color or a border color
– in the Page settings: SEO, OG and RSS can be bound to the Collection fields
– inside of every block of custom code (inside of a Collection List on static pages, or anywhere in a Template page)
– How to browse all elements in the Navigator to spot purple ones? In the Navigator tab, select the Body element, keep Alt pressed while you press Arrow Down as many times as needed to browse all element. This will unfold the complete tree view of your site.
– pay attention to the error message, they sometimes contain clues to where a binding is conflicting
– if you ever run into a case where you think all the bindings are deleted, but they are not, try to do a full version save of the site, by pressing cmd+shift+s in Mac or ctrl-shift-s in Windows. If that does not allow you to delete the collection element(s) then send an email to email@example.com.