Bulk noindex for a particular category

One category on my site is syndicated content and the syndicator does not want me to index any of it as part of our agreement. I can click “noindex” on each post, but that would be over 2,800 posts and would take too much time. I’ve already got the category set to noindex, but because these posts are linked to from other posts, they end up being spidered anywy. I was hoping that there is either a plugin or a hack that would allow me to select one category and apply noindex to each individual post underneath it. I tried doing this with “Edit Custom Fields” and a some code in my header.php file that someone recommended, but it didn’t seem to do the trick.