Hi,
Once you have validate the "guest checkout", HikaShop has created an account for the user and because it is not link with the Joomla user ; there is no "logout" button.
But that guest account is link with the current session.
We are currently performing a refactoring of the checkout system and we will see to improve that "guest checkout" management.
But for the moment the best is to clear the session, by deleting the session cookie.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.