Why does my Roku express 4K remote also control the Roku express+ Roku. how can I fix it if there’s a solution?
You can't prevent a roku IR remote from controlling all roku's it has line of sight to. You'll have to abandon the IR remote and buy a wifi roku remote and pair it to the express, that's if the express isn't 3700, which only takes IR remotes.
Or hide the Express 4k+ or put a piece of black tape over the front of it.