Good question! I'm surprised that the information does not appear to be available.
Shirlea, I went back and looked at my own history of proposed merges. I see that when I proposed a merge, I was notified when someone postponed it (converted it to an Unmerged Match). I imagine that is the same for rejections, but I don't find any in my own experience.
Meanwhile, I'd think the number of people who can reject a match is fairly limited, just the profile managers. Maybe someone else will have a better idea, but I'd suggest contacting them (starting with the most active).