My FIZZ modem has created two Wi-Fi networks. How do I delete one of them?

One of our networks is the main one that we created when we set up the Fizz modem and it is password protected. The other network arrived from out of nowhere (we didn't set it up) and it does not have a password. It looks like our neighbours are using this 'free' network, which is eating up our bandwidth and preventing our main, password-protected network from working properly.
How can I delete this second Wi-Fi network, or at least add a password to it so that people outside of our houshold cannot use it?
Thanks so much!
(The second network is named EVduty-EVC30-16455, in case that is a name that Fizz uses often)


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