What an annoying problem.
This is the solution.
Had my Sprint gold 64gb S6 since the 12th and have noticed that when I turn off WiFi and get close
to a place I’ve connected before it will automatically connect to it if anybody can help me
figure how to get it to stop this?
This is the Sprint Connections Optimizer. To turn off,
you go to Settings>>Mobile Networks>>Connections Optimizer,
where you can uncheck “Connections Optimizer”
Also, be sure to deselect connecting to partners wifi automatically.
I noticed my S6 was still connecting automatically to Xfinity the
other day because this was still checked.