hide an entire subdomain from search engines.

Hi experts, thank you in advance for anyone that can shed some light on this.

I design and develop small websites for clients and use sub-domains from my own website (www.nextcode.com.au) as development areas.

I was hoping to find a way to completely hide these sub-domains from search engines as the clients don't want content and designs that haven't been finalised seen by the general public.

It appears placing code in the robots.txt file is the best option for this (within each sub-domains folder) however I am not sure what to be putting here to hide the entire sub-domain or if this is even the best option.

I was hoping someone could advise of the best option for ensuring that sub-domains I want hidden are in fact hidden from search engines.

Thank you to anyone that is able to shed some light on this request.

Richard.