Lobo (DC Comics) was so violent that he was banned from Hell, and he caused so much damage in Heaven that he was kicked out of there as well, rendering him unable to die.
Any human who uses a Death Note (Death Note) is permanently barred from both Heaven and Hell.
King Piccolo's victims (Dragon Ball) spend eternity suffering in a state of limbo, never resting in peace.
Horcruxes (Harry Potter) are objects and/or living beings that house pieces of a person's soul, preventing them from moving on to the afterlife as long as the Horcruxes exist...
...though in Voldemort's case, his soul was badly mutilated due to his making too many Horcruxes, which rendered him unable to move on and trapped him in Limbo for eternity.
Hiruzen Sarutobi (Naruto) using the Dead Demon Consuming Seal to entrap his own soul and that of his victim's inside the Shinigami's stomach, rendering them unable to enter the afterlife until released.
Anybody who falls victim to a Cenobite (Hellraiser) can find no release, their soul tortured and perverted into one of their kind for eternity.
Anybody killed by Freddy Krueger (A Nightmare on Elm Street) will have their soul held captive by him, increasing his own power in the process.
Metatron (Supernatural) has prevented Heaven-worthy souls from entering Heaven after he sealed it.
Gaede (Valkyrie Crusade) can prevent others from entering on any afterlife and send them back to the land of the living.
Hellgate (Valkyrie Crusade) as the embodiment and guardian of the hell's gate, can either prevent others from entering on hell, or send them there.
The Brothers Death (SCP Foundation) barred SCP-1440 from the afterlife as punishment for using the prizes they gave him (The Elixir of Life, Cards of Fate, and the Sack of the All-Death) to humiliate them at every turn.