Consult with a psychologist about this.  If the child has obsessive behaviors due to psychological issues, then certain actions on your part could trigger or make the actions worse.  For example, taking something away may escalate the child into a much more destructive behavior.  Work closely with mental health professionals to make sure you are addressing the behavior in the best possible way.

That being said, here are a few ideas that may help if you get the go ahead from mental health:

• Use the child’s obsession to help you work on something (ex: recalling past events from the child’s favorite show/movie)
• Use the child’s obsession as a reward during sessions (“first work, then ____”)
• Minimize attention to the obsessive behaviors if they aren’t interfering with learning (they may actually be helping the child stay focused and in control)
• Allow time for the child to engage in obsessive behaviors and then set a timer for the time when the child is not allowed to do so (start small)
• Show the child how to use some form of language to request the obsession or obsessive behavior so that she doesn’t feel so out of control when you take it away