To remove GRUB entries, we'll need to edit the file /boot/grub/menu.lst. You can do this by using Alt+F2 and then typing in the following command:
gksu gedit /boot/grub/menu.lst
Now that we've got the file open, scroll down to the bottom of the file where it says "End Default Options", and you'll find all the menu entries for the various kernels in here. You can just select and delete the ones you want.
Save the file, and then the next time you boot up you'll see a much nicer set of options.
No need to delete those lines to simplify the GRUB menu. The easier way to keep only newest lines is to open the menu.lst (gksu gedit /boot/grub/menu.lst), find "#howmany=all" and change it to "#howmany=1″. Then run "sudo update-grub" to update changes. Next time, only newest kernel lines will be shown.