Prevent a base themes css file from loading

You can use wp_deregister_style in your child theme. Like this.

function remove_unwanted_css() {
    wp_dequeue_style( 'bootstrap' );
    wp_deregister_style( 'bootstrap' );

    wp_dequeue_style( 'bootstrap-custom' );
    wp_deregister_style( 'bootstrap-custom' );
}
add_action( 'wp_enqueue_scripts', 'remove_unwanted_css', 20 );

Similarly you can deregister other unwanted css too. Just add wp_deregister_style in this function.