How to unlink previous employees Apple ID?

Yes.

The Employees can log in to http://icloud.com and remove the devices from their account by deleting them from the Find My iPhone option there.

You can follow the steps provided in the following support document:

Turn off Find My iPhone Activation Lock - Apple Support

Alternatively, if you can produce legal documentation i.e the purchase receipt of the iPads specifying the company as the purchaser, the company's representative can take them to an Apple store and request they be restored.

As part of the exit interview, verify that the device as been deactivated.

Verify that your ios device is free of the activation lock.

How to check if the activation lock is active on a device

https://www.icloud.com/activationlock/

employee should follow this procedure...

What to do before selling or giving away your iPhone, iPad, or iPod touch

http://support.apple.com/kb/ht5661

https://support.apple.com/en-us/HT201351

----

more work, but use dep and an mdm to establish the company not the employee as the owner.

"This document offers guidance on some important considerations for getting the most out of your iOS deployment." Covers: Prepare your infrastructure. Set up devices. Configure and manage devices. Deploy apps and content. Plan for support.

https://www.apple.com/business/docs/iOS_Enterprise_Deployment_Overview.pdf

Quick overview of zero touch deplayment

http://www.apple.com/education/it/dep/

Consider DEP, Device Enrollment Program. Use VPP, Volume Purchase Program. Select an MDM, Mobile Device Management. Read the Google doc below and nsdjoey write up.

Overview of ipad & iphone deployment.

It's education focused.

http://www.apple.com/education/it/

Writing you own app for your business:

https://developer.apple.com/enterprise/

MDM -- Mobile Device Management

https://developer.apple.com/videos/play/wwdc2015-301/

For a comparison see this page:

http://www.enterpriseios.com/wiki/Comparison_MDM_Providers

VPP -- Volume Purchase Program

http://www.apple.com/business/vpp/

http://www.apple.com/education/it/vpp/

Using Apple Configurator 2

See ccsdtech writeup on page 5

See weinsteinbevit plus Meraki MDM on page 5

https://discussions.apple.com/thread/7302890?answerId=29798579022#29798579022&tstart=0#29798579

Alternatively, if you can produce legal documentation i.e the purchase receipt of the iPads specifying the company as the purchaser, the company's representative can take them to an Apple store and request they be restored.

Or just call and then email your Apple-contact. 😉