Un-indexing an entire website – temporarily

Hi all,
Long time listener, first time caller.
I need to unindex the entire US version of my work website. The reasoning behind it is that all the content is exactly the same as the UK site and won’t be changed for the foreseeable future, and I do not want to get penalised by Google for having two identical sites.

But I still want to be able to index it once the site has been updated with its new content.

Any ideas on how to do this? Any help is much appreciated.

Thanks in advance!