How to Remove /feed URLs from Google's Index

[M_D_Golden_Peak]

Hey everyone, I have an issue with RSS /feed URLs being indexed by Google for some of our Wordpress sites. Have a look at this Google query, and click to show omitted search results. You'll see we have 500+ /feed URLs indexed by Google, for our many category pages/etc. Here is one of the example URLs: http://www.howdesign.com/design-creativity/fonts-typography/letterforms/attachment/gilhelveticatrade/feed/. Based on this content/code of the XML page, it looks like Wordpress is generating these:

<generator>http://wordpress.org/?v=3.5.2</generator>

Any idea how to get them out of Google's index without 301 redirecting them? We need the Wordpress-generated RSS feeds to work for various uses.

My first two thoughts are trying to work with our Development team to see if we can get a "noindex" meta robots tag on the pages, by they are dynamically-generated pages...so I'm not sure if that will be possible. Or, perhaps we can add a "feed" paramater to GWT "URL Parameters" section...but I don't want to limit Google from crawling these again...I figure I need Google to crawl them and see some code that says to get the pages out of their index...and THEN not crawl the pages anymore.

I don't think the "Remove URL" feature in GWT will work, since that tool only removes URLs from the search results, not the actual Google index.

FWIW, this site is using the Yoast plugin. We set every page type to "noindex" except for the homepage, Posts, Pages and Categories. We have other sites on Yoast that do not have any /feed URLs indexed by Google at all.

Side note, the /robots.txt file was previously blocking crawling of the /feed URLs on this site, which is why you'll see that note in the Google SERPs when you click on the query link given in the first paragraph.

[M_D_Golden_Peak]

I tried many different htaccess file codings (such as recommended here), but they didn't work. Had to succumb to using the outdated Meta Robots plugin by Yoast, which can add the "noindex" code to the http header of /feed/ URLs. But, at least it's a solution: http://wordpress.org/plugins/robots-meta/. Hopefully this helps someone else.

[M_D_Golden_Peak]

I believe I found the solution: implement an x-robots-tag into the HTTP header of the various feed URLs. But, I need some help with creating the code to place in my .htaccess file. Any takers?