Remember that the Linux kernel is monolithic, so many drivers are compiled into it when it is built, though now, there are many modules available that can be loaded at runtime. Given this information, it is recommended that you usually keep at least the last kernel version on your machine, incase the new version has drivers that conflict with your current hardware.
As bobince mentioned, you can simply remove the versions of the kernel you do not want. Since hard disk space is in abundance (usually) in modern machines, I would recommend simply removing the entries from the bootloader menu configutation rather than uninstalling the kernels.
You can do this by opening the Terminal and typing "gksudo /boot/grub/menu.lst", finding the lines for the old kernel versions, and either deleting them, or placing a # (comment marker) before the Title line.