Limit ONE link per post

Take it as a starting point (not tested):

/**
 * Limit links inside the content.
 * 
 * @param (string) $content
 * @return (string) $content
 */
function wpse30754_limit_content_links( $content )
{
    // delete all <a> tags in the post
    $content = preg_replace( "/<a[^>]*><\\/a[^>]*>/", '', $content );

    return $content;
}
add_filter( 'the_content', 'wpse30754_limit_content_links' );

What’s not yet implemented is preserving the first link. You’ll need that on yourself.