I have a Bluetooth speaker that I use occasionally, but not often. A few weeks ago I decided it was time to upgrade my Mid-2011 iMac from Mavericks, to Sierra (not High Sierra). Ever since then, the BT speaker connects itself as my Mac's audio device every time the machine wakes up from sleep. This is very annoying... if I disconnect a device, I expect it to stay disconnected unless I manually connect it. Why does this keep happening and how can I stop it?
It's probably because it's been paired with the mac. Unpair the device.
Remove a Bluetooth device from your Mac If you want to remove (unpair) a Bluetooth device from your Mac's device list, follow these steps: Choose Apple menu > System Preferences, then click Bluetooth. Hover the pointer over the device you want to remove, then click the button Cancel button that appears next to the device's name.
After you remove a Bluetooth device, you'll have to repeat the pairing process if you want to use it again.