I recommend posting an idea for a "Send Operational Email" permission. It might just be because it's the end of the day, but I can't think of a way to restrict a user from sending one unless you get Workspaces, build your operational emails in a specific workspace, and then deny the user access to that workspace - which wouldn't be very practical for most users.
Grant is right, this is not an option on Admin>Roles.
You can restrict users from Sending or Approving emails, so you might want to do that until you train the team to properly use Operational.
Thank you Grant and Josh. That is what I thought, but I just wanted to get confirmation from some experts. I will definately take the recommendation to post it as an Idea.