I refer to your letter / email of (date). ). I / We appreciate your interest in our company.
I regret to inform you we are not currently able to accede to your request for an informational interview at this time, due to … (see optional paragraph).
… Optional paragraph
… pressures of work / lack of resources / facilities it is not possible for us to conduct interviews of this type.
current commitments of time for our staff.
… business circumstances.
I regret we have been unable to assist you at this time,