Only allow one item in cart at a time

Add the following in functions.php:

add_filter( 'woocommerce_add_cart_item_data', 'woo_custom_add_to_cart' );

function woo_custom_add_to_cart( $cart_item_data ) {

    global $woocommerce;
    $woocommerce->cart->empty_cart();

    // Do nothing with the data and return
    return $cart_item_data;
}

Source: https://pinakibisi.wordpress.com/2014/07/02/woocommerce-one-item-in-cart-at-a-time/