Returning guest customers

Hi,

A guest checkout won't generate a user account and the checkout will accept an email address regardless of how many guest checkouts it has done in the past, as long as no user account is linked to the email address.
However, if you try to guest checkout with an email address which already has a user account, then the system will refuse the guest checkout.
Also, if you don't want to offer login capability during the checkout, you need to make sure you didn't activate the setting "Allow registration after guest checkout" of the HikaShop configuration. Otherwise, after the guest checkout, the user will get a link to be able to register and, if he does, he will get a user account.

So either the email address that was used by the customer notifying you of the problem either had created a user account on your website before you turn off the user registration in Joomla, or you have activated the "Allow registration after guest checkout" setting and the customer registered after his purchase. So please check on that.