1- How can I remove items from the admin menu? => Go to Nop.Web > Areas > Admin. You will find sitemap.config file there. It contains all default menus of nopCommerce admin. You can remove or add menu items in it.
2- ow can I remove nopcommerce new (news?) from admin? => Go to Nop.Web > Areas > Admin > Views > Home. Open Index.cshtml and remove from line 25 to 32 (for v4.3).
3- How can I change powered by nopcommerce in website and admin (Just in development and testing phase)? => Browse All settings (advanced) (https://yoursite.com/Admin/Setting/AllSettings) page from your browser. Search settings by name storeinformationsettings.hidepoweredbynopcommerce. Update the settings value as True.