I probably do not understand what you mean by "unallocated", but you can use the command line utility (run it as root) "fdisk" (or Window's Computer Management Control Panel applet) to repartition part of the USB stick and leave other parts unallocated (that is, not in a partition). Insert the stick, unmount it as necessary, and use fdisk on the stick's device file (/dev/sd?) just as if it were a hard drive, which is exactly how the OS treats it. Delete the existing partition, which probably includes the entire capacity of the stick, then create a new partition that is smaller than the capacity. Any parts of the stick that are not in a partition will be unallocated. Once you save your new partition scheme and exit fdisk, you will probably need to format your new partition(s).
Note that this will destroy all the information on the USB stick. Save it first if you want to keep it.
Last edited by bwayson; 05-07-2009 at 03:54 PM.