The user can negate the healing factors of other beings and essentially render others more vulnerable to damage. Users who deal lethal attacks to others will place their victims in critical conditions due to the victim's body being unable to heal the damage.
Akame's (Akame Ga Kill!) Shingu, Kiri Ichimonji, inflicts wounds that will never heal.
Taisuke Kanō (Alive: The Final Evolution) can use the heat from his Destruction and Rebirth to destroy cells, preventing regeneration.
Kami (Dragon Ball) had some level of negating regeneration, as he managed to prevent Goku's tail from regrowing again.
Gae Buidhe (Fate/Zero) is a mythic spear from Norse legend said to be capable of inflicting wounds and injuries incapable of ever being healed be it by magic, science or otherwise. It's literal name meaning "the golden rose of mortality".
Sesshomaru's (InuYasha) Bakusaiga releases an infectious wave of deadly corrosion that prevents regeneration.
Meliodias (The Seven Deadly Sins) using Enchantment: Hellblaze, black flames that also nullify an immortal's regeneration.
Gae Bolg (TYPE-MOON) is a cursed spear that inflicts wounds that do not heal until the spear is destroyed.
Wounds made by Harpe (TYPE-MOON) can only be healed according the laws of nature.
Lavawight's (AD&D) possess Blazefire, an attack that inflicts damage that can never be healed by any means, including divine intervention.
Agent Zero (Marvel Comics) secretes a powerful corrosive that directly attacks an opponent's healing factor, which not only prevents the opponent from healing, but actually worsens the wounds.
Crota's (Destiny) Presence alone can negate any Guardian's abilities to heal.
Zeref's (Fairy Tail) Predatory Death kills all that comes into contact with it, but those that survive it cannot be healed or naturally heal unless it can be cured by a special type of magic.
Whoever is touched by Ian’s (Marchen Awakens Romance) Darkness ÄRM, Iron Maiden will be unable use Holy ÄRM and heal themselves. As a side effect, Ian also is unable to use Holy ÄRM.
The evil power of Ghost ÄRMs (Marchen Awakens Romance) prevents any kind of magic from reversing the pain they bring to their user.