1. Open the iOS Settings app and go to “Bluetooth”
2. Wait for the device list to populate, then tap on the (i) button next to the Bluetooth device you want to disconnect
3. Tap “Disconnect” to disconnect the iOS device from the targeted Bluetooth device, whatever it is
This is particularly useful for Bluetooth peripherals with multiple devices that use them. For example, if you want to quickly disconnect an external Bluetooth keyboard for use elsewhere, maybe another iPhone or iPad or Mac, without having to forget the device entirely.