The guest user may not be enabled by default - this depends on the version of WebLogic Server which you are running. In any case,
you should never default to guest user if your user is defined in the WebLogic security realm.
To disable the guest user, do the following.
Go to the Change Center of the Administration Console, click Lock & Edit. Select your domain and select Compatibility Security > General. Now select the Guest Disable checkbox. You will need to reboot your server for this change to take effect.