There's a lot of ways to do it from manaully editing your configs and removing the files to purging them from apt. I recommend the later though.
You can see which is your current version by typing:
You can see the kernels you have installed by typing:
dpkg -l | grep "\-2.6."
You can remove them individually by typing:
apt-get remove --purge package-name-revision
I suggest leaving the two most recent kernels (the one you're running and previous one.)