Comment 3 for bug 1410272

In general, apt-get purge the linux-image-generic linux-image linux-headers linux-headers-generic linux-generic packages.

Then you can delete some image files from /boot/ by hand; once one or two are deleted, use apt-get purge to delete the corresponding packages (linux-image-*-generic, linux-headers-*-generic). Be sure to keep at least two bootable kernels.

Once you've finished removing packages, re-install the linux-image-generic linux-image linux-headers linux-headers-generic linux-generic packages to ensure you continue to get security updates.