Excluding categories from RSS

https://site.com/feed/?cat=-X
where X equals the category ID number.
Conversely, if you did https://site.com/feed/?cat=X (no minus sign) you’d get a feed of ONLY that category.

Thanks, but as I stated I need to exclude more than one category at a time. The method you described appears to only work for the exclusion of a single category.

I am not entirely sure but try adding $cat=1,2,3,6; (this example excludes cats 4 and 5) or whatever is appropriate below the following lines in the rss php.
$blog = 1;
$doing_rss = 1;

FYI – I wanted to exclude a certain category (51) from being in the RSS feed for any user. Modifying wp-rss.php to

$blog=1;
$doing_rss = 1;
$cat=-51;

did the trick!