I mean, you can cite an item from your library and then delete it in Zotero. The citation will remain in the document and will show up in the bibliography, and you can even continue to cite it elsewhere in the same document (by selecting from the Cited section of the citation dialog search results). But you won't be able to edit the metadata if necessary, and, well, this isn't really the way people use Zotero, which exists to help you build up a research library. But if you really just want a citation in one Zotero-generated document without it remaining in the library, that's how you'd do it.