Manta Rays are giants that, regardless of their size, (as much as 7m!!) pose no risk to people. This remains in contradiction to Sting Rays which can deliver a deadly sting.
Humans in truth are the best danger to the Rays and they are now considered susceptible due to overfishing. They typically get tangled in fishing nets as they are unable to swim backwards. As soon as knotted, they drown due to their consistent requirement of flushing oxygen abundant water over their gills.