You can block it via Mac Filtering in the WiFi settings of the Gen5 modem. You can find the MAC address of the phone, while it's connected, in the Devices section of the Status box on the lower right of the WiFi Settings Home page.
If you know how to get to the WiFi settings in the Gen5 modem, the MAC filtering table is in under Advanced Setup > Wireless > MAC Filtering Table. To get to the WiFi settings, either click on the WiFi settings in the lower left of the System Control Center or go directly here. Then use admin as the password to enter the WiFi settings.
To filter the phone's MAC address, first Enable MAC Filtering in the MAC Filtering Table, then click Save Settings at the bottom. Then enter the MAC address of the phone and click Save Settings again. The phone should then immediately disconnect from the internet.
I hope this helps.