First, run the following command to remove outdated linux headers:

# apt-get autoremove

Do not remove the current kernel, which you identify with:

$ uname -r 

List all installed kernels on your system:

# dpkg --list | grep linux-image 

Remove selected kernels:

# apt-get purge linux-image-2.6.xx-xx-server

After removing a kernel, update-grub is called automatically. Do call in again manually:

# update-grub2 

Reboot your system.

Check the following directories:

/usr/src
/lib/firmware/2.6.xx-xx-server
/lib/modules/2.6.xx-xx-server
/var/lib/initramfs-tools/2.6.xx-xx-server
/boot

Kept back packages