Modifying metadata for xwiki pages

Hello everyone,
I observed that google indexed my xwiki which is not acceptable.
To hide it I created robots.txt with
“User-agent: *
Disallow: /”

but that still after re-crawling it appears in search results.

In one of topics people suggested to add new entry in each page metadata:

Is there any way to modify it through xwiki or any config file?
I went through properties and cfg files but cannot see any way to do this.

Thanks in advance for answers/help,
Artur