I have many subdomains for my main site in order to help my rankings. However, I have recently read that subdomains with similar content may be considered spam. The content is different in each subdomain, but I do not want to take a chance of having the subdomains labeled as spam and then hurt the entire site. The subdomains do not have enough value to take this chance.
I do not want to simply take down all of the subdomains. Can I use the robots.txt file to prevent the SE's from further viewing the subdomains? If so, do I put a robots.txt file in each of the subdomains?