White rhinos can run 30 mph. How could a grizzly win? A rhino would easily crush its ribcage with its horn in one charge.
100 people could kill a grizzly bear, assuming they are not cowards. many would get mauled, but many people would get chances to strangle the grizzly, eventually the grizzly would be too tired to keep throwing them off.
No one person could even reach around the neck of a white rhino and the skin is too thick for any unarmed attack to work really so I would say no number of unarmed people could kill a white rhino