During initial setup, remote was set to control tv on/off. This is too complicated for 90 y.o. mother who also has cable attached to her tv. What is process to stop Roku remote from controlling the tv?
There's no way that I know of to get it to forget the setup. You have to go through the procedure under Settings->Remotes & devices->Remotes->[voice remote]->Set up remote for TV control and just choose the wrong brand of TV and lie when it asks if it's working. Some different brands use the same remote codes, so don't be surprised if the first "wrong" brand you choose still works.