Welcome!
Is it just OTA? Or all SD channels?
You must be talking about HD OTA channels. When these channels broadcast SD programs, your local broadcasters add those black bars on each side, and these bars are broadcast as part of the image. Since neither VOOM STB, nor your TV set can change geometry of HD signal, you are stuck with that! The only solution for you is to switch to 480i mode when you are watching those upconverted programs and then your TV set should be able to zoom/stretch... Not very convenient, I know, but I doubt there is anything else you can do.