No indexing of certain categories

[Live24x7]

I am putting up a new forum, where two categories will be populated using relevant rss feeds.

I want to enable 'no index' metatag for just these two categories to avoid duplicate content issue on my websit.

if i Plan to use robots.txt.
What should i write in robots.txt ?

will this do the trick ?

Disallow /nameofcategorytobeexcluded/*

I am using pretty url and forum is installed in the root folder.

Or is there any other way to achieve this ?

Thankyou