how to desauthorise computers

Hi,

As everybody knows you can syncronize your iphone with 5 computers. In my case, the one at work and my computer at home. Problem. I was on holidays and the technicians changed my computer Three times!!!, every time download itunes but when they changed the computers they did not desactivate the synchronization between my iphone and the computers. I would like to know a way to desauthorise those computers (there is no option to get information of the computers because all the information was erased). Please help!!!