Keep in mind with the file blocker that they can change mymail.pst to mymail.mail, then associate .mail files with the mail client. As long as the pst file never hits the filer, it won't stop the actual file type from getting to the filer. The native file block only looks at extension, not the file metadata to read the actual type.
Smart users will simply set the mail client up to create a file other than pst and continue to use them.
Exactly. You will get all manner of replies from ONTAP guru's telling you to do this and do that when what the customer wants is to prevent PST files being used. The way to do this across the infrastructure is to set group policies, through the local machine or centrally in AD and block them there. Block the file on the controller and you've bloked them in one place - except of course you haven't. So block the functionality, not the file extention.