I already know that in some comics he cannot die as a result of Death barring him from her realm. But has he ever been defeated in combat by one person or a group of people? I'm only wondering because they'll have to get rid of him somehow in the Marvel Cinematic Universe.
Yes, he has been defeated.
- In Avengers Assemble #1-8 he returned to earth but the Avengers along with the Guardians of the Galaxy teamed up and beat him.
- He has also been stopped by Adam Warlock in Avengers Annual #7
- In the Infinity storyline Thanos is trapped in a pocket limbo of stasis by his son (Infinity #6)
- (On Earth 616) he has been defeated by the Mangog (Thor Annual Vol 2 #2000)
- He has been beaten by Drax on the Annihilation series.
Here is his punch: