You have to work at it a little, but you can, in fact, libel someone in a fiction story. Here are two excellent articles that tell you how you could libel a person in your story and, more importantly, how to avoid doing so. I’ll get out of the way and let you get right to this valuable information.
First Amendment Center (This article mentions a libel suit brought against the makers of the movie Hardball. Scroll down to Muzikowski v. Paramount Pictures Corp. and you can see how that came out. And here is how the Sandlot case ended.)