The open WiFi hotspot on the Echo and Echo Dot is only enabled when it is in setup mode and is required to allow it to be configured via the Alexa app until you have it connected to a router. Once it has been setup and is connected to your router, it will no longer be accessible in that way (if you look on your phone it should no longer even show as a WiFi hotspot) and so nobody should be able to use it to access your router or PC. It will only be enabled again if you need to put it back into setup mode (by holding down the action button on the top of the Echo / Echo Dot).