For redirect non admin users to home page and disallow back end access in WordPress, you need to simply copy and paste the following code in your theme’s functions.php file.

function redirect_non_admin_user(){
    if ( !defined( 'DOING_AJAX' ) && !current_user_can('administrator') ){
        wp_redirect( site_url() );  exit;
    } 
}
add_action( 'admin_init', 'redirect_non_admin_user' );

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To disable WooCommerce Payment methods based on cart item quantity, you need to simply copy and paste the following code in your theme’s functions.php file.
Here I’m unsetting the COD when cart item quantity limit reached 10.

add_filter('woocommerce_available_payment_gateways', 'disable_payment_gateway_based_on_quantity', 10, 1);
function disable_payment_gateway_based_on_quantity( $available_gateways ) 
{
    $qty_limit = 10;
    $limit_reached = false;
    foreach(WC()->cart->get_cart() as $cart_item){
        if($cart_item['quantity'] > $qty_limit ){
            $limit_reached = true;
            break;
        }
    }
    if($limit_reached){
        unset($available_gateways['cod']);
    }
    return $available_gateways;
}

You can use the same code for disable WooCommerce Payment methods based on cart item quantity. Change quantity and payment method slug for your needs.

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