This removes both the excerpt and the longer encoded content from the RSS feed.

remove-feed-content.php

<?php

/**
 * Removes the content from the RSS feed and replaces it with a custom message.
 *
 * @param string $content
 *
 * @return string
 */
function yst_clean_rss( $content ) {
	if ( is_feed() )
		return 'This is a new post on yoast.com, Whoopity Doo!';
	return $content;
}

add_filter( 'the_content_feed', 'yst_clean_rss', 90, 1 );
add_filter( 'the_excerpt_rss', 'yst_clean_rss', 90, 1 );