You can choose who is able to see when you were last online in Whatsapp in the privacy settings. The options are: everyone, my contacts, and none. If you say none, you can't see other people's Last Seen. I think if you have someone in your contacts and they have WhatsApp, you would have to set it to none if you didn't want them to see it. Or maybe block them inside the WhatsApp app, assuming that didn't affect anything outside the app. I'm not sure what the everyone is for. Maybe group chats or something.