Short answer
HubSpot's KB: for content already indexed, add a noindex meta tag to the head HTML. For content not yet indexed, add it to robots.txt. Don't combine them: robots.txt prevents search engines from seeing the noindex tag. robots.txt only covers domains connected to HubSpot, and can be edited for all domains or one.
1. Sitemap
Sitemap settings in my demo portal.
2. If a page isn't indexed
Check for a leftover noindex, and request indexing in Search Console. See Search Console.