Stop iPad from automatically using iPhone data? We have employees who are issued both an iPhone and an iPad. 
The iPads are on Verizon and have unlimited data. 

The iPhones are on AT&amp;T and have limited data. 

The problem happens when they are using the iPads and lose service. The iPads automatically switch on the iPhones hotspot and start using it. This is causing the users to go over their AT&amp;T data limit when using their iPads.

Is there any way to stop them from automatically switching over to the iPhones data?
The feature you are describing is called "Instant Hotspot," though you will not see that phrase in the UI of any of your devices.  Instant Hotspot allows Macs and iOS devices to use the "Personal Hotspot" feature of one an iOS device to provide internet connectivity to a Mac or another iOS device, without the need to switch on the Personal Hotspot feature on the host iOS device.

In order to use Instant Hotspot, each device must be signed into the same iCloud account.  That's it.  There is no switch to enable or disable.  Just sign into the same account.

It seems impractical to ask your users to use separate iCloud accounts for their phones and iPads... so your best solution may be to call up AT&amp;T and ask that the Personal Hotspot feature be disabled on the phones... thus ensuring that the iPads will stick to their native data connections.