When you hear the thud of hooves, they're usually from a horse--not a zebra.
As a community college instructor, I have to say there are a lot of zebras out there.
Ok, so let's say that he's got a disability of some sort, or that he had a bad experience with alien abduction, or whatever. Does it really matter the reason he's being like this? The OP still has to decide to stop reacting to this guy, and his disruptions still have to be dealt with (or not, as is appropriate to the situation )
It depends, at my college, whether or not the student is registered with our Disability Resource Center (in the U.S.). If so, then I can contact the DRC for help with managing the student's behavior.
If he/she is not registered, I have two choices per my dean - I can either kick them out as a disruptive student or allow them to remain in class. Many instructors are reluctant to kick out a student for behavior that is not dangerous or threatening. Since the instructor has already talked to the student to no avail, odds are the behavior won't be modified if the student stays in class.
The issue of behavior modification is why a disability or no disability matters - there many ways I can deal with student issues in a classroom, but honestly most are not effective on students with certain disabilities.
To HonorH, yes that's definitely one of the perks, but most professors use it sparingly...and I second the ignore advice.