Say, you want to remove kernel 2.6.19-gentoo-r5, you should first remove the source and the compiled image:

root# rm -rf /usr/src/linux-2.6.19-gentoo-r5
root# rm -rf /boot/System.map-genkernel-x86-2.6.19-gentoo-r5
root# rm -rf /boot/initramfs-genkernel-x86-2.6.19-gentoo-r5
root# rm -rf /lib/modules/2.6.19-gentoo-r5/

Then you would need to remove the entry for 2.6.19-gentoo-r5 in the GRUB menu:

root# vi /boot/grub/grub.conf