To unpair a Bluetooth accessory, go to Settings > Bluetooth, find the device you want to unpair, and tap the More Info button , then Forget this Device. This removes the accessory from the list of available Bluetooth devices.
To add the Bluetooth accessory to your device again, place it back in discovery mode and repeat the steps to pair your device.
Unfortunately you can't force pair it. Unless you buy a new bluetooth speaker that allows multiple connections or comes with a software control panel such as the BlasterAXX. It is understandable that once a connection is established another device should not be able to terminate this connection and force a new one.
Receivers that allow only one connection are doomed to reconnect to the last device connected and the only way to resolve it is to disconnect the other device. I had a number of receivers and bluetooth speakers in the past and I always ran around the house to find the device that is connected to the speaker. If the receiver is not configured by the factory to allow multiple connections then you can't do much. Unless you want to crack open the receiver and start messing around with the chips (which I am sure you wouldn't like to do)
The ones I used had a pairing button, that once I pressed it a new search began and I was able to connect a different device. You should check if there is a similar option.
The logical way to proceed in the future, is to turn off your bluetooth on the device, after you are done, so that the receiver is free for a new connection. Now, if you want to find which device is connected, you can always use your phone to scan for nearby devices and go check every one of them if it is connected to the receiver.
Another way is to configure your devices so that they wouldn't automatically connect to the receiver, but only when you desire to. I am sure that every single device, or computer you are using has the option to connect automatically or not.
I understand this is not what you would like to hear but there is no other way. That's a common problem with devices that don't have a control panel available. If you are not willing to spend some money on a new device then try any of the things I mentioned, and I would suggest to have the receiver free for connections when you are not using it.