I started in an Assistant Manager position at my job two years ago. At the time, I had a co-assistant manager who was incredibly difficult to work with. We worked in customer service, and he was consistently rude to customers, getting attitude on the phone and in person. He had it out for our supervisor, constantly complaining about her to her superiors, undermining her, and generally causing tension and unease in our team. He was let go about a year after I started as part of a department restructuring, but I'm told he was also talked to about his performance issues at the time.
Flash forward to now, I received a promotion and am currently hiring for my vacated position. My former coworker applied. My team and I are not interested in hiring him back, but now I'm tasked with writing a rejection email that's slightly more delicate than most. This coworker still works with the organization on a part-time basis in a different department that interacts with mine, so I will still see him and have to work with him. My supervisor told me to "be careful with it, but go ahead and be human." Any advice on what I should say in order to not rock the boat?