Q. Can I restrict Cortana use in the enterprise?
A. Yes. There are a number of group policy options to control access to Cortana and also the ability to associate Microsoft accounts (MSAs) with AD accounts (which is a requirement to use Cortana).
To control is MSAs can be used see article https://technet.microsoft.com/en-us/library/jj966262.aspx?f=255&MSPPError=-2147217396
To enable to block Cortana see article https://msdn.microsoft.com/library/windows/hardware/dn904962.aspx#Experience_AllowCortana (note this is a great article that covers many different policies and which SKUs of Windows they work in).
For group policy locate Computer Configuration -> Administrative Templates -> Windows Components -> Search and then scroll down to Search and find "Allow Cortana". Edit the policy and then set to Disabled or Enabled to disallow or allow.