There isn't an option to disable it with a setting, but you can can remove the data-preview-url property from the <xf:editorrow /> tag in the template. If you don't give the editor a URL for previews, it won't have the preview button/option.
The "description" field has a preview URL, so you get the preview button, but further down, "Related resources" and "FAQ" use the editor but without a preview URL.