Hi @chun,
The guest options are a little tricky.
The option “Do nothing but block guests from purchasing products beyond the limits” will just block guests from buying products above the current limits. For example, if the “Mode” option is set to Product quantities and the limit is 1, the guest won’t be able to purchase a quantity bigger than 1. Now, with the “Mode” set as Orders, the Do nothing but block guests from purchasing products beyond the limits option won’t help so much.
Maybe you’ll need to use the “Identify guests by IP address” instead. That way, if you have the option from WooCommerce “Accounts & Privacy > Guest checkout > Guest checkout > Allow customers to place orders without an account“, the guest IP will be saved on his purchase and he won’t be able to buy again if using the same IP.
