Front page showing up on google search

I got a multisite.
I use the main site to display the posts of the sub-sites using MU Sitewide Tags plugin and have turned on " Discourage search engines from indexing this site" since day 1.

Nonetheless, Google search shows posts from the main site.

Though it redirects readers to the relevant sub-site, it seems like it is indexing two sub-sites for similar content. Because sometime I also see the relevant subsite on google search results.

Can someone help me with this.

Thanks very much.