If you don't want to unpair the device, but just want to disconnect from it, Apple have 3 methods:
Turn the device off and hope it doesn't reconnect back to your iOS device when you turn it back on again.
Turn Bluetooth off on your iOS device, you obviously don't need to stay connected to more than one Bluetooth device.
A very natural and simple process Apple have thought about carefully is that you simply walk your iOS device away from the device, about 5-20 metres, until it is out of range. You don't even need to unlock you iOS device to perform this step! Once out of range your iOS device will disconnect and you can walk safely back into range again.
I hope this helps, Apple have clearly thought these processes through and have delivered easy to understand and intuitive methods for disconnecting iOS from your Bluetooth device.