How to permanently stop Samsung Galaxy S5 from keep connecting to Wi-Fi?
Disable the WiFi from the Notification Menu to stop the device from connecting to the WiFi network.


Drag down from the top of your device screen, the notification menu and a range of quick action buttons will appear.
Assuming your phone is connecting to the WiFi, the 'WiFi' quick button will be a green colour signifying it is enabled. Touch the button once, and wait for it to turn light grey. This means your WiFi is off.
Pull up the notification menu, and look at the status bar. There should be no WiFi symbol visible and your phone isn't connected to a WiFi network.