I am assuming the fan has a remote you can use, and it is likely an IR controller. The ceiling fan must use the same IR codes as the Roku. I have had that happen with a Roku remote and a Vizio TV. The Roku remote would control the TV, and it wasn't even the TV the Roku was connected to.
With overlapping IR codes, there isn't much that can be done, other than attempting to shield the emitter end of the remote when using it so the fan IR receiver doesn't see it. Easier said than done sometimes.
Depending on the model Roku you have, you might be able to switch to a WiFi Direct remote, so it doesn't emit the Roku IR codes any longer. I switched my Premiere remote with a WiFi remote, and it no longer affected the Vizio TV.
Dan
Roku Community Streaming Expert
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