This is how I do it might be easier ways.

open terminal and see what kernel you are using.

Code:
uname -a
then see what is installed

Code:
dpkg-query -l | awk '/linux-image-*/ {print $2}'
remove the ones that you do not want

Code:
sudo apt-get --purge remove <Kernel>
then
Code:
sudo update-grub

MAKE SURE YOU DO NOT DELETE THE KERNEL THAT YOU ARE USING