Hmmm, I would suppose your BT adapter on the computer is not disconnecting (By this term. I mean it is not turning totally off.) When I turn off my BT on a PC it is definitely OFF but can easily be turned back on.
I am glad to know you did see the two previous threads on this sort of issue. Look again, and read where I described how to have Echo clear ALL BT devices able to connect to it. It is in the Alexa app Settings menu, see Your (named) Echo Online, tap that and tap Your (named) Echo, and not far down the list is Bluetooth, tap it and select to "Clear all paired devices". Your computer will not pair with the Echo again unless you re-pair the two again. This should really stop all this nonsense you are having. Of course, any device you had paired with Echo will also be un-paired, too. A little more affect than is intended, so I suggest you do this at least once for this instance, but also figure out how to have your computer completely turn off BT to prevent it going back to its old trick.
I have found that getting my computer under control is most effective to prevent the issue you are having of unintentional connecting of BT between PC and Echo from happening.