how do I stop iPhone from automatically connecting to bluetooth devices?

Thanks for taking the time to consider this, but this ins't a useful answer - it's a classic case of "the tale wagging the dog"; changing your behaviour to accomodate a design fault, instead of fixing the design fault.

It would make a great deal more sense if I could choose which Bluetooth device to connect to, instead of creating an automatic connection - this is a design fault in the iOS

Firstly, I have seven bluetooth devices, that as I said in my original post, I use at different times for different purposes through out the day for different reasons. This means that I may want to use the same device, at different times during the day, but use another device or devices at a different time in the day between using the original device. My iPhone is quite happy to decide for me which device it will connect to (although, as I mentioned in my original post, some items are more likely than others). I don't think that I'm unusual in having more than one Blue tooth device paired with my iPhone.

Pairing & unpairing devices is a cumbersome process on the iPhone , and it also implies that you have time to unpair a device (such as when a phone call comes in - but perhaps you're smarter than me and know when your phone calls are coming in before the phone rings; I have to wait for my phone to ring, before I know that there's a call. then I have to answer it to discover that my iPhone has decided to pair with a device without asking me). Unpairing a device is not an instant process, it takes a bit of time to do, you need to access and go into various menus on the iPhone (I've just tried on my iPhone, and counted seven menu function interactions required to unpair a device).

Following your line of logic, of unpairing a device when it's not needed, would mean that I have to unpair every device after I've finished using it, just in case my iPhone decides to automatically connect to it at a later point in time (such as when my iPhone rings). I'm not sure if you've done much pairing of iPhone devices, but it's a cumbersome process, that often doesn't work on the first or second attempt, but you'd suggest that every time I want to use a device I go through this process, and make sure that I unpair the device when I've ceased using it?

So, I'll have to spend my days pairing & unpairing devices, to stop my iPhone automatically connecting, instead of changing the design functionality to reflect a common user experience - having multiple Bluetooth devices connected, and allowing the user to determine which device to connect to.

Sorry, thanks for your help (and for taking the time to consider my comments), but your answer (that unpairing a Bluetooth device is the only way to prevent this problem), highlights that I think that there is a design flaw in Apple's implementation of the Bluetooth system in their iOS - apple could develop a system to allow the user to choose the device to connect to, when there are multiple Bluetooth devices present, a situation that will become increasingly common.