A LONG time ago, I worked for a company that had a very strict policy against any employee, even managers, from providing references for former employees, regardless if they were good or bad.
I had a guy working for me that was a nightmare. I don't have enough room to list the problems he caused. If anyone deserved a negative reference, this was the guy. So when his next potential employer called me for a reference, I used the same ploy - "Company policy precludes me from telling you what a worthless pieces of human trash he is, and also prevents me from warning you that you should never hire that guy. I'm sorry I can't be of more assistance."
I felt very good that I help prevent another company from enduring the problems this guy caused.