If your Roku is using a WiFi Direct remote (does it have a headphone jack, or is it controlling a Roku Stick) then you could put some tape over the IR window of the TV so it doesn't respond to the AC remote. I had the same issue with a Roku remote controlling a Vizio TV. Since I was using the TV as a computer monitor and never turned it off, I simply covered the IR window on the TV so the Roku remote couldn't hit it.
Of course, if your Roku is using an IR remote, then you need to leave that window uncovered. Your only recourse would be to buy a Roku Voice remote and pair it with the TV, so you can block the IR window (since the voice remote doesn't need IR to work).