Hide the comments form in a specific category?

<?php
if(!in_category('5')) {
// If not in category with the ID 5...
   YOUR COMMENTS CODE
}
?>

NOTE: You might need to use is_category as appose to in_category..

Or create a file called category-5.php and exclude the comments code (as in, just leave it out)…

In both cases change the 5 to the ID of the category…

Rel:
http://codex.wordpress.org/Template_Hierarchy
http://codex.wordpress.org/Conditional_Tags