in addition to the above answers you could try to remove the kernel image + system.map + config files manually. grub looks for those files in the /boot directory.
remove any files not related to the kernel version(s) you use. if you are using only the latest version, let's say 3.2.0-25, it should be safe to remove all 2.6.* and 3.0.* versions. so you type ls -alF /boot/2.6. to see what's there and remove those. same with all the other versions except the one(s) you are using.
NB: when updating the kernel it may be good policy to keep the version that previously worked best for you