Prevent WooCommerce to add more than 1 item in cart

wc-block-add-to-cart-sl2.php

<?php
/**
 * Simple & concise Solution
 */
function op_block_add_to_cart() {
    wc_add_notice( sprintf(
        /** translators: %1$s is cart url, %2$s is checkout url */
        esc_html__( 'You cannot purchase more than 1 item at a time. %1$s or %2$s', 'op' ),
        '<a href="' . esc_url( wc_get_cart_url() ) . '">' . esc_html__( 'View cart', 'op' ) . '</a>',
        '<a href="' . esc_url( wc_get_checkout_url() ) . '">' . esc_html__( 'checkout', 'op' ) . '</a>'
    ), 'error' );

    return WC()->cart->is_empty();
}
add_filter( 'woocommerce_add_to_cart_validation', 'op_block_add_to_cart' );

wc-block-add-to-cart.php

<?php
/**
 * Forget about 'woocommerce_add_to_cart_validation' filter hook
 * that's why created this solution :P
 */
function op_block_add_to_cart( $cart_item_key ) {
    if ( count( WC()->cart->get_cart() ) > 1 ) {
        // Remove latest item from cart to prevent side effect
        WC()->cart->remove_cart_item( $cart_item_key );

        // Show notification to user
        throw new Exception( sprintf(
            /** translators: %1$s is cart url, %2$s is checkout url */
            esc_html__( 'You cannot purchase more than 1 item at a time. %1$s or %2$s', 'op' ),
            '<a href="' . esc_url( wc_get_cart_url() ) . '">' . esc_html__( 'View cart', 'op' ) . '</a>',
            '<a href="' . esc_url( wc_get_checkout_url() ) . '">' . esc_html__( 'checkout', 'op' ) . '</a>'
        ) );
    }
}
add_action( 'woocommerce_add_to_cart', 'op_block_add_to_cart' );