Have you ever or do you have the ability to refuse to take an assignment because it so sickened you?
I haven't ever, but my editors are reasonable people. I am sure if I voiced my disgust, they would find another reporter who is willing to do it.
However, I find most of my stories on my own. It is not the editors' job to find the news. If I do run across something I think is news worthy, but am too disgusted to report on it, i would just send the idea to my editors and ask them to find some one willing to cover it.