598How to make a content item used as 404 page, or any other item, to be unsearchable/documentation/tutorials-english/73-configuration/598-how-to-make-a-content-item-used-as-404-page-or-any-other-item-to-be-unsearchable.html
it is good to make item state to be Archived
For Search view -- in your search MENU item (search module also requires a menu item, and uses it for configuration) do not add the content type of the item to searchable types
For category-like views that also have text search -- assign the item to a category that you have configured to be non-linkable (* see below) and also make sure the category is top level (no parent) and it has no direct menu items to it
(*) go to FLEXIcontent Advance Route plugin and select not to create links to the category