package management - Unable to resolve unmet dependencies because /boot is full and apt install -f can't finish

I'm trying to install openvpn however I am running into a problem where Ubuntu is complaining about unmet dependencies:

[sudo] password for keith: 
Reading package lists... Done
Building dependency tree       
Reading state information... Done
You might want to run 'apt-get -f install' to correct these:
The following packages have unmet dependencies:
 linux-image-extra-4.4.0-71-generic : Depends: linux-image-4.4.0-71-generic but it is not going to be installed
 linux-signed-image-4.4.0-71-generic : Depends: linux-image-4.4.0-71-generic (= 4.4.0-71.92) but it is not going to be installed

 openvpn : Depends: libpkcs11-helper1 (>= 1.11) but it is not going to be installed
E: Unmet dependencies. Try 'apt-get -f install' with no packages (or specify a solution).

When I try to run apt-get -f install I'm met with these errors:

dpkg: error processing archive /var/cache/apt/archives/linux-image-4.4.0-71-generic_4.4.0-71.92_amd64.deb (--unpack):
cannot copy extracted data for './boot/System.map-4.4.0-71-generic' to '/boot/System.map-4.4.0-71-generic.dpkg-new': failed to write (No space left on device)
No apport report written because the error message indicates a disk full error

If I run df -h I see the below:

Filesystem                    Size  Used Avail Use% Mounted on
udev                          208M     0  208M   0% /dev
tmpfs                          46M  5.2M   41M  12% /run
/dev/mapper/temptus--vg-root  8.3G  6.7G  1.2G  86% /
tmpfs                         228M  4.0K  228M   1% /dev/shm
tmpfs                         5.0M     0  5.0M   0% /run/lock
tmpfs                         228M     0  228M   0% /sys/fs/cgroup

/dev/sda2                     473M  470M     0 100% /boot
/dev/sda1                     511M  3.6M  508M   1% /boot/efi
tmpfs                          46M     0   46M   0% /run/user/1000
/home/keith/.Private          8.3G  6.7G  1.2G  86% /home/Keith

As you can see, /dev/sda2 (/boot) is 100% used. I tried following Can't clean a full /boot because of unmet dependencies, which does free up space in /boot, however when I remove old kernels and finally run apt-get -f install, the space required always increases more than what I cleared.

For example, if I manage to run apt-get -f install before removing kernels it tells me I need 66.4mb of free space. But if I remove a kernel (4.4.31 for example), it increases to 122mb of free space and wants to reinstall whatever kernel I removed.

Is there a way to forcefully remove kernels and have Ubuntu not reinstall after running sudo apt-get -f install?

Thanks!