This KB will show how to Prevent users from creating PST files using Group Policy
First follow the guide below to download and install the Outlook 2013 or 2010 ADMX administrative template file
http://www.ntweekly.com/?p=8602
Once done create or edit a new group policy and go to:
Microsoft Outlook 2013 -> Miscellaneous > PST Settings – > Set the following policies
Enable – Prevent users from adding PST’s to Outlook Profiles and/or Prevent using Sharing-Exclusive PST’s and set Enabled and select ‘No PST’s can be added’ inside the menu
Enable – Prevent users from adding new content to existing PST files