add_action( 'wp_enqueue_scripts', 'deregister_cf7_javascript', 15 );
function deregister_cf7_javascript() {
    if ( !is_page(15) ) {
        wp_deregister_script( 'contact-form-7' );
    }
}
add_action( 'wp_enqueue_styles', 'deregister_cf7_styles', 15 );
function deregister_cf7_styles() {
    if ( !is_page(15) ) {
        wp_deregister_style( 'contact-form-7' );
    }
}