No index for an entire site

Nick's answer to add robots.txt might not stop Google from index, it just says stop crawling but Google might index - official link here.

Adding meta noindex is too risky if you are working with a team that is not aware of implications of noindex tag, if it is pushed to production env.

<meta name="robots" content="noindex">

I would recommend you to go with password protecting your website, that seems to work the best and easier to manage, with team members who are not big on SEO. And does the job of stopping access to users as well as search engines.