How to prevent iPad from using Personal Hotspot?
On the iPad, in Settings &gt; Wifi, tap the 'i' in a circle icon next to the listing for the Personal Hotspot, then tap the Forget This Network tab.
You should make absolutely sure the Personal Hotspot is turned off. Looking at Settings &gt; Personal Hotspot, it is pretty unambiguous.
You should also check to see if Instant Hotspot has been enabled:

Instant hotspot
You can use Personal Hotspot on your iPhone to provide Internet access to your other devices without additional setup. You'll need to sign in to iCloud using the same Apple ID as your iPhone. Also, your cellular provider needs to provision your Personal Hotspot.
To get Internet access from your iPhone:
On your iPad or iPod touch, go to Settings &gt; Wi-Fi on your other iOS device and select your iPhone or iPad that has a cellular connection.
On your Mac, go to the Wi-Fi menu at the top of your screen and select your iPhone or iPad that has a cellular connection.

From this Apple Support Page