Hi,
1 - As same as @luceos said, first you have to exclude users from sitemap so google can't find users page in easy way.
2 - Then you have to edit your robots.txt file and Disallow user pages. So by doing this, if Google find some user's profile link in topics or somewhere else, that will stop Google from indexing user's pages. For robots.txt you can use this commands:
User-agent: *
Disallow: /u/*
