How to prevent the sub pages from getting indexed in Google?

Hi,

Actually subpages are treated by the internal WordPress pages system, so Multipage follows the standard WordPress Behavior. Anyway if you install a SEO plugin, like Yoast SEO, Multipage will fully support rel meta next and prev, e.g.

<link rel=”canonical” href=”https://www.textsite.com/2015/12/hello-world/2/&#8221; />
<link rel=”prev” href=”https://www.textsite.com/2015/12/hello-world/&#8221; />
<link rel=”next” href=”https://www.textsite.com/2015/12/hello-world/3/&#8221; />

More info on: https://support.google.com/webmasters/answer/1663744?hl=en