Ubuntu sometimes updates itself with a new Linux kernel version. Such kernel updates also create new entries in the GRUB bootloader screen which is displayed at bootup. I am not doing any kernel development and am confident enough that Ubuntu folks are updating me to safe stable versions. So, if I do not have any problems with the new kernel version, I find it irritating to see older Linux kernel entries cluttering up the GRUB screen.
Ubuntu now uses GRUB2, which is significantly different from the older GRUB. To remove Linux kernel entries from the GRUB bootloader screen, just un-install the related Linux kernel packages using the Synaptic Package Manager. And voila, the GRUB entries for that are auto-magically removed!
For example, to remove the GRUB boot entries of Linux kernel v2.6.31-20, I searched for 2.6.31-20 in Synaptic. I found the packages
linux-image-2.6.31-20-generic. I un-installed them, and their boot entries are gone! 😊
Tried with: Ubuntu 9.10 (Karmic Koala)