@Ayu,
No – there’s no ‘take my site down’ button, unfortunately! However, we do have a small maintenance mode workflow. Although this would use bandwidth, it’d be minimal – particularly as the simple ~5kb site would be cached.
If you’re using Netlify DNS and a custom domain, sure! You have the power to tinker with these or delete the zone entirely. If you’re using your .netlify.app URL, this won’t be possible. If you’re not using Netlify DNS but you are using a custom domain, you can alter the records at your provider.