You can reduce the tile size as Spaceland notes (I ususally use 42 on my multi core rigs) but you cannot control processor usage. In my experience, most 3D apps grab everything that they can at render time an Carrara is no exception. Making a render take longer is usually not on most peoples minds. ;)

If you have more than one computer, you can set up the second (and/or third) as part of a network render to offload some processing but the main machine still needs to be part fo the render and re-assembly of the image or animation.