Remove or release connected device from Gateway G36

You can try clearing the browsing history and restart the modem for a minute then check the modem interface or GUI connected device list if still you see the old device shows on the list then you can only remove device connected to the modem network by changing the network name and re connecting only the device that you want to be shown on connected device list after resetting modem this is the only way to remove the device showing under connected device list on the modem interface or GUI. Please follow the steps mentioned on the below article to change the details.

http://arris.force.com/consumers/articles/General_FAQs/G34-G36-Factory-Reset

http://arris.force.com/consumers/articles/General_FAQs/G34-G36-Changing-the-Wi-Fi-Name-and-Password