If you would like to log out of your current Stocard account to start fresh with a new unregistered account, you can simply go to the Stocard settings and tap on "Account". Here you will find the option to "Create a new account".
Please note that by selecting to "Create a new account", you will start with a completely empty card list. If you were using a registered account before, you can always restore the cards in that account by logging back in. However, if you were using Stocard without a registered account before choosing to create a new account, it won't be possible to restore the cards that were in your Stocard app before you create the new account.