Can a walrus kill a polar bear?

Definitely. A polar bear is much bigger and stronger than a grizzly bear, when it has sharper claws, stronger jaws, and is a better killer.
Large male polar bears are larger than male grizzlies. However, since the grizzly is a race of the brown bear, which has specimens as large as polar bears, it's hard to say. The race called grizzly bear is somewhat smaller than the coastal race called Kodiak bear. There exists video of a grizzly bear driving polar bears away from a garbage dump.