With the Polar Bear you just stick out your tongue and make faces at it, call it's mother bad names and generally piss it off.
When it charges you, you jam the spear into the ground and let the Polar Bear impale himself. Easy peasy.
Also works on Grizzly's too. I'd not attempt it on Cape Buffalo, unless you got a real stout stick, maybe even steel.
Do let me know how this works out for you.