Block all subdomains in Google search

[Ruby707]

I have a website-domain.com & it has many sub-domains like sub.domain.com, sub1.domain.com, etc. I don't want the subdomains to be indexed & crawled in Google.

I have added robots.txt file in each subdomain: User-agent: *
Disallow: /

It was still showing in google when searched for domain.com.

I had to add each subdomain in Google webmasters tool & used the remove URL setting.

Is there any other method wherein it won't be necessary to add subdomain each time in webmasters tools?

Please guide.
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