If they're doing it with EFS then do the following:
1. Open the GPO
2. In the Group Policy Object Editor, expand Computer Configuration, expand Windows Settings, expand Security Settings, expand Public Key Policies, and then click Encrypting File System.
3. Right-click Encrypting File System, and then click Properties.
4. Clear the Allow users to encrypt files using Encrypting File System (EFS) check box, and then click OK.