This one liner can purge all old kernels except for the one currently running.
sudo apt-get remove --purge $(dpkg -l 'linux-image-*' | sed '/^ii/!d;/'"$(uname -r | sed "s/\(.*\)-\([^0-9]\+\)/\1/")"'/d;s/^[^ ]* [^ ]* \([^ ]*\).*/\1/;/[0-9]/!d')
Does the trick all the time. 
Can free up hundreds of MBs too if many still remain.
Always take caution removing old kernels, and double check the packages being removed are actually kernel packages (I've not had a problem with the command above).