If your opponent has ten poison counters, he loses the game, so you just need to get your opponent to zero, then deal ten more damage to him. He'll take it in the form of poison counters, and lose. It will buy him time - maybe he was almost dead and suddenly you need to get him to 0 then deal ten more damage - but he's not invincible.
After reading this it got me thinking; and correct me if I have missed something.
If you are at 1 life, and you get hit by a 5/5 creature, do you get 4 poison counters, or none, as you are now at zero life? (Excluding trample of course.)
That being said, once you're at zero life, can you gain 1 life and repeat this?