Well, when connected to internet using the Live TV input of the RokuTV, that is really only providing the Guide Info (the TV scheduling for the various channels). If you are disconnected from internet, should not decrease the available channels acquired from your Antenna. (you just won't be provided any info from the guide as to what is playing on the channel or schedule info.) You may just need to get a more powerful antenna to connect to the TV to receive more channels.
If you do not mind having to enter your password each time you want to use the streaming capabilities of your RokuTV, you can just navigate to Settings..System..Advanced System Settings..Reset Network Connections..Network Reset.
This will reboot your TV and disconnect it from your network (and thus, internet). You then just select the Live TV tile to watch your OTA channels but will most likely find this an undesirable way to watch as no program guide will be available and you will be limited to using the up and down arrows to switch channels (no guide=no quick scrolling through channels). This will be same result no matter how you disable the internet to the TV.
Another way to disable internet to specified devices is to change some settings within router similar to Parental Controls or managing DHCP clients. (Use your RokuTV MAC address, not IP address which can change, when defining rules). Of course you will have to login to your router admin each time to enable and disable access as required. (Unless you have a newer router that has an accompanying App to control connected devices such as a Pause or Block feature).
Are you needing to disable internet to save on streaming bandwidth? I am sure any bandwidth used while watching OTA is minimal as, again, just used for guide info.
.
