A. PST files are a burden in many environments. They're hard to manage, hard to perform discovery on, and prone to loss when stored on local machines. Many companies now use archiving solutions at the mail server, such as the Exchange 2010 archiving functionality, to avoid the need for user PST files.
To phase out PST files, take a two phase approach. First, you can stop users from writing to PST files but still allow them to read from then. You can then totally block PST files.
You make these changes using two policy settings that are found in the Outlook Group Policy template under User Configuration, Policies, Administrative Templates, Microsoft Outlook <version number> Miscellaneous, PST Settings. The settings are called Prevent users from adding new content to existing PST files and Prevent users from adding PSTs to Outlook profiles and/or prevent using Sharing-Exclusive PSTs.
If you don't want to use Group Policy, you can achieve the same result using registry changes. The keys are found under HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\<version, e.g. 12.0 or 14.0>\Outlook (set value DisablePST to 1 to disable the use of PSTs) and HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\<version, e.g. 12.0 or 14.0>\Outlook\PST (set value PstDisableGrow to 1 to disable writing to PSTs).