16.04 - How to free space on a full /boot partition?

Stuck running Ubuntu 16.04 LTS on a Lenova desktop with a small boot partition now with 0(zero) bytes after many upgrades. Currently at 4.4.0-71-generic but new upgrades fail. Have tried suggestions from many "solutions" but still can not upgrade.

None of the following free-up space:

sudo apt-get clean
sudo apt autoremove --purge
sudo apt-get autoremove --purge
sudo purge-old-kernels
sudo apt autoremove
sudo apt-get purge linux-image-4.4.0-67

Have also purged all the other old linux-images that do not have dependencies.

Any new suggestions?