There are two ways you can remove them. The easy way is to open up a command line, and type 'sudo cp /boot/grub/menu.lst /boot/grub/menu.lst.bac' and then 'sudo gedit /boot/grub/menu.lst'. Scroll down until you find the kernel entries. One might look something like this:
Then you go through and comment the lines you do not want out. To do that, you put a # in front of them, so it will look like this:
title Ubuntu 8.04.1, kernel 2.6.24-19-386
kernel /boot/vmlinuz-2.6.24-19-386 root=/dev/sda3 ro quiet
Save and exit.
# title Ubuntu 8.04.1, kernel 2.6.24-19-386
# root (hd0,0)
# kernel /boot/vmlinuz-2.6.24-19-386 root=/dev/sda3 ro quiet
# initrd /boot/initrd.img-2.6.24-19-386
MAKE SURE YOU LEAVE AT LEAST ONE KERNEL AND ONE RECOVERY KERNEL!
The other way, which actually isn't that hard, has you uninstalling the kernels. You can do this either through Synaptic or the command line. If you're using Synaptic, you find the packages that contain the extra kernels, and you remove them, again making sure you leave at least one kernel.
If you prefer to use the command line, the command would probably be 'sudo apt-get purge linux-2.6.24-19-386 linux-2.6.24-19-server' and so on. (The naming might be different, you can look for them using 'apt-cache search server | grep 1.6.24' or something like that.)
The first way may not be permanent if upgrades come, but is a great way to see if everything works with them 'removed'. The second way will remove them unless you have a package that depends on them.
Hope this helps!