Prevent creation of PST files but allow opening them

I’ve had a request to use group policy to prevent the creation of PST files in Outlook 2013 but allow existing PST files to be opened. I’ve found a group policy setting that disabled opening and creating them, but can’t identify a setting or combination of settings to fulfil the request.

Is anyone aware if this can actually be done? Even if there’s a combination of registry values I can set via GPP that’ll do.