I hate manually cleaning out all of the old kernels here is a script to do it for you:
dpkg -l 'linux-*' | sed '/^ii/!d;/'"$(uname -r | sed "s/\(.*\)-\([^0-9]\+\)/\1/")"'/d;s/^[^ ]* [^ ]* \([^ ]*\).*/\1/;/[0-9]/!d' | xargs sudo apt-get -y purge
This was stole directly from here.
For good measure I run the following after to be sure the grub config is good:
Reboot to make sure everything is OK.
Which much restraint I avoided including several bad un-popped popcorn references.