An obsessive focus on one favorite subject is a hallmark characteristic of Asperger’s Syndrome.
Students often become fixated on this subject to the complete exclusion of everything else, and their academic experience and social relationships can suffer greatly as a result.
Don’t try to eradicate this interest. You won’t be successful, and the conflict you’ll create by trying will lead to a host of other problems. The student is passionately interested in the subject, so the question is: how can that passion be controlled? How can the interest be diversified and connected with other subjects? Perhaps your student is fixated on sci-fi films, for example. While remaining supportive of the interest, you might assign the reading of some sci-fi short stories, and begin to connect to the study of English, language arts, and literature. Or perhaps the student could draw his own sci-fi aliens, label the physiology of the alien (skeletal system, organ systems, etc.) and thereby connect to the subjects of biology and life sciences. Pay attention to what the student responds to, and be creative.
While it is important to expand the student’s interest, it won’t do much good to always deflect that interest towards schoolwork. Once in a while, let the student show off his or her talents a bit. Some extra credit for a short presentation on the subject can be highly rewarding, motivating, and enjoyable for your student with Asperger’s Syndrome. Just be sure to clearly communicate your expectations about the time limit beforehand.