192.168.100.1 or 192.168.0.1 if you have a leased combo unit. Then, depending on what model you have, follow the UI to wifi settings.
Alternatively if you have a leased all-in-one modem/router combo unit, you can call Spectrum and have them set it to bridge mode and disable wifi.
If you are using a separate router, tell us what brand and model it is and we may be able to direct you to the correct menu.
Remember that if Spectrum provided your router, it is free but use of the WiFi is NOT FREE. I believe their current WiFi surcharge is $5 per month. If you find the WiFi On/Off setting to be locked in the User Iinterface, that's why.