Welcome to Android Central! Go to Settings>Connection>Location>Improve Accuracy and look for the Scanning option. Make sure Bluetooth Scanning is turned off.
This is often insufficient. It seems Google Services for Play also might trigger involuntary BT on.
So, go to Google Play Services -> Change system settings (permission) -> Allow access <<-- change to No
This solved BT popping back on for me.
It happens to me also and what I did is just go to system settings and check devices connected. There you can find the option for Bluetooth and more...just turn it off.