Yes, it can be disabled... System performance will not be impacted, but your modem will log a statecode indicating that the wifi is disabled.
First go to your modem at http://systemcontrolcenter.com or http://192.168.0.1 then click Wifi Settings
You will then be asked to enter a password, by default it is "admin" with out the quotations. Once logged in, select a wifi network. Then uncheck SSID Enable, then click Save Settings.