We’re unable to find the amount of force needed to rip off a human head. However, we found the amount of force needed to crush a human skull.
According to Sciencealert.com, human bone is incredibly sturdy. It is stronger than steel and concrete of the same mass. The human skull encases the most important organ of the human body, the brain, and has evolved to be as strong as possible to withstand trauma.
Lenny Bernstein from the Washington Post spoke to neurosurgeon Tobias Mattei and reported that a skull fracture requires 500 kg of force.
It would be possible for a man who weighed 500 kg to fracture a skull by stepping on it but, according to Mattei, it would be “impossible [for a man] to break [a skull] with his hands even if 90 percent of the 235 kg were biceps muscles.” Moreover, “It would be almost impossible … to ‘blow up’ the head’s top from inside… No explosion would be seen. The eyes of the victim would be pushed backward some few inches. That’s it.”
Cynthia Bir, a biomedical engineer at the University of Southern California, confirmed this assessment writing that:
”There is no way to get the head to ‘explode’ by applying pressure from the eyes. You would need to create pressure inside the cranium. Even if you could generate pressure by squeezing the outside of the head, once the cranium is breached at the orifice where the eye nerves enter, this pressure would be greatly diminished."
Kyle Hill at the Slate asked the question about a similar stunt in the Star Trek movie. Quoting a bike-helmet study published in the Journal of Neurosurgery: Pediatrics, he wrote that 235 kg (520 pounds) or 2,300 newtons of force would be needed to crush a human skull, almost twice as much force as human hands could possibly muster.
You may recall from high school physics that Force = Mass x Acceleration, so something quite heavy has to be travelling very fast to generate enough force to crack a human skull.
Of interest, discover the effects of blast pressure on the human body, shared by CDC.gov.