dpkg --list | egrep -i --color 'linux-image|linux-headers'

returns:

ii  linux-headers-4.15.0-70                    4.15.0-70.79                                    all          Header files related to Linux kernel version 4.15.0
ii  linux-headers-4.15.0-70-generic            4.15.0-70.79                                    amd64        Linux kernel headers for version 4.15.0 on 64 bit x86 SMP
ii  linux-headers-4.15.0-72                    4.15.0-72.81                                    all          Header files related to Linux kernel version 4.15.0
ii  linux-headers-4.15.0-72-generic            4.15.0-72.81                                    amd64        Linux kernel headers for version 4.15.0 on 64 bit x86 SMP
ii  linux-headers-generic                      4.15.0.72.74                                    amd64        Generic Linux kernel headers
rc  linux-image-4.15.0-29-generic              4.15.0-29.31                                    amd64        Signed kernel image generic
rc  linux-image-4.15.0-55-generic              4.15.0-55.60                                    amd64        Signed kernel image generic
rc  linux-image-4.15.0-58-generic              4.15.0-58.64                                    amd64        Signed kernel image generic
rc  linux-image-4.15.0-60-generic              4.15.0-60.67                                    amd64        Signed kernel image generic
rc  linux-image-4.15.0-62-generic              4.15.0-62.69                                    amd64        Signed kernel image generic
rc  linux-image-4.15.0-64-generic              4.15.0-64.73                                    amd64        Signed kernel image generic
rc  linux-image-4.15.0-65-generic              4.15.0-65.74                                    amd64        Signed kernel image generic
rc  linux-image-4.15.0-66-generic              4.15.0-66.75                                    amd64        Signed kernel image generic
rc  linux-image-4.15.0-69-generic              4.15.0-69.78                                    amd64        Signed kernel image generic
ii  linux-image-4.15.0-70-generic              4.15.0-70.79                                    amd64        Signed kernel image generic
ii  linux-image-4.15.0-72-generic              4.15.0-72.81                                    amd64        Signed kernel image generic
ii  linux-image-generic                        4.15.0.72.74                                    amd64        Generic Linux kernel image

sudo apt --purge remove linux-image-3.12.0-27-generic

The purge removes packages and any configuration files are deleted too.

On newer system all obsolete kernels and headers should automatically be flagged as not being needed, and thus can be purged with the following single command:

sudo apt --purge autoremove

returns:

Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following packages will be REMOVED:
  linux-headers-3.12.0-27* linux-headers-3.12.0-27-virtual*
0 upgraded, 0 newly installed, 2 to remove and 0 not upgraded.
After this operation, 64.7 MB disk space will be freed.
Do you want to continue [Y/n]? y
(Reading database ... 218493 files and directories currently installed.)
Removing linux-headers-3.12.0-27-virtual ...
Removing linux-headers-3.12.0-27 ...