1- Get the category # of the category you want “excluded” form your RSS feed
(note: in my example my category was “movies” which had a category number of 12)

2- open & edit  the “functions.php” file

3- add the following code below to the bottom of your functions.php file
(also make sure you replace the -12 with the # from step 1, add a – sign to it also)

4- Save functions.php file

4- want exclude MORE? then add more categories with commas between

 function myFeedExcluder($query) {
  if ($query->is_feed) {
    $query->set(‘cat’,’-12′);
  }
 return $query;
 }
add_filter(‘pre_get_posts’,’myFeedExcluder’);