First install the app called MobileGuard. Its a cool app. I’ll recommend it even if you want to keep whatsapp. Next, open MobileGuard, and go to Clean Spam > Memory Cleaning > Clear in Batch > Select WhatsApp. Now select the ‘Off’ option. This should close the app and all its services
Keep in mind that whatsapp will start again when you restart your phone. If you want to turn that off, go to Boot Mgr in Mobile Guard and disable it.