The one person to “kill” Thanos in the Marvel universe was Adam Warlock during the
Infinity Gauntlet cycle. Thanos first killed Warlock and sent his spirit into the Soul Gem. Then when the Avengers and Fantastic Four were being trounced by Thanos, Spider-Man made contact with the Soul Gem, which released the spirit of Adam Warlock who turned Thanos into stone, before returning to the soul gem.
But no one stays dead for long in comics! Later, Mistress Death brought Thanos back to life.
During the Heart of the Infinite story, Thanos became more powerful than The Living Tribunal. But when the universe was about to die, Warlock convinced Thanos that to save the universe, he had to sacrifice himself. Thanos agreed and killed himself
@Peter Popoff @Bryan Danielson