yes,
you should manually set a DNS server on your WiiU
Or use a wrong gateway on the Wii U to ensure it has no internet connection.
This will block access to the updates on the WiiU, but not on the 3DS
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3. Use TubeHax DNS on your Wii U
3.1. Go to Wii U Menu/System Settings/Internet/Connect to the Internet
3.2. Input your router's SSID and password (typically WPA2-PSK AES)
3.3. Input settings as follows:
IP Address: Auto-obtain
DNS: Don't Auto-obtain
Primary DNS: 107.211.140.065 (TubeHax DNS)
Note: This will leave you unable to access YouTube
Secondary DNS: 0.0.0.0
Note: This should make your Wii U fail to connect to the internet if TubeHax fails.