Essentially, another immortal could throw gods to Tartarus and imprison them there, which is quite analogous to killing them, since no-one could escape that utmost prison. Another point to consider is the hardest mineral called adamant. read more
There was a creature called the Ophiotaurus that was part bull and part serpent, born from Chaos at the same time and Gaia and Ouranus, and whoever killed it and burned its entrails was granted with the power to kill gods. read more
From my view, In Greek Mythology it's impossible to kill a god. I mean the gods are immortal so they don't age or die. But they can be crippled like what happened with (Uranus), and faded like most of the gods.. people just forgot that they existed, I mean they can be reduced to almost nothing or imprisoned like the Titans who were imprisoned in Tartarus. read more