In general the spirit only God (one above all maybe) can actually kill the ghost rider spirit, and Mephisto says he is unable to kill them. Many things can harm them though such as: very high powered tech, hellfire, Angels or fallen angels, and sufficiently powerful magic.
some feats i like aside from sparing world war hulk from the beatdown of his life, a gang of ghost riders from over time took down a Rogue arch-angel who ascended the throne of 'god' himself (who was reality warping prior), even thor, dr voodoo, punisher and spiderman could sense the battle transpiring in heaven.
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