The free WiFi that BT offers is basically a small amount of your given bandwidth is allocated to anyone who uses/has access to BTs free WiFi, you also have access to other people’s free WiFi when you are out and about.
The OP was asking how to turn off this section of the WiFi and mentioned it was interfering with his network but in reality yes it does create 2 channels on the same as your primary channels but interference would really only be an issue if someone was using it and that those channels are on the same channels as his other networks.
This is why with mesh you should try to not have the same channels overlapping but in reality many users with BT equipment need not worry or even notice a drop in performance.
So the OP either moves to a third party router that will not broadcast the free WiFi or they opt out of this insuring that those channels are off and not simply hidden.
It could be they just don’t want that signal broadcasted as I doubt it will be an issue with performance.
