Wordpress - How do I remove a pre-existing customizer setting?

Late to the party but this will do the trick:

$wp_customize->remove_control('blogdescription');

You want to remove just that control, not entire section as suggested above.

Remove a pre exising customizer setting in wordpress theme with this code.

add_action( "customize_register", "ruth_sherman_theme_customize_register" );
function ruth_sherman_theme_customize_register( $wp_customize ) {

 //=============================================================
 // Remove header image and widgets option from theme customizer
 //=============================================================
 $wp_customize->remove_control("header_image");
 $wp_customize->remove_panel("widgets");

 //=============================================================
 // Remove Colors, Background image, and Static front page 
 // option from theme customizer     
 //=============================================================
 $wp_customize->remove_section("colors");
 $wp_customize->remove_section("background_image");
 $wp_customize->remove_section("static_front_page");

}

I found out the WP_Customize_Manager class has a function called remove_section(). In your function hooked to customize_register you can just do:

    $wp_customize->remove_section('nav');
    $wp_customize->remove_section('static_front_page');

You can find the ID of the section (i.e. 'nav') if you inspect the accordion title bar of the section. Look at the ID of the containing <li> tag and it's the portion of the string after "customize-section-". I.E.:

<li id="customize-section-static_front_page" class="control-section customize-section">

-- the ID is "static_front_page"