There are some spells which cant reduce or block the damage of Poison Sting, Dispersion and Vhoul Assassin's Envenomed Weapon.

1. Pick Venomancer and level up Poison Sting
2. Create an enemy hero from the first list below and level up the named ability
3. Cast named ability if needed
4. Attack the hero affected by the ability

The following abilities cannot reduce or block Poison Sting/Dispersion/Envenomed Weapon damage:
1. Bristleback's Bristleback
2. Centaur's Stampede (aghs upgraded)
3. Ursa's Enrage (aghs upgraded)
4. Medusa's Mana Shield
5. Nyx Assassin's Burrow (aghs spell)
6. Nyx Assassin's Spiked Carapace
7. Abaddon's Aphotic Shield
8. Templar Assassin's Refraction
9. Treant Protector's Living Armor

On the other side, these one can reduce/block it:
1. Abaddon's Borrowed Time (on self and on allies when upgraded)
2. Oracle's False Promise
3. Io's Overcharge
4. Kunkka's Ghostship
5. Spectre's Dispersion
6. Legion Commander's Duel (aghs upgraded)
7. Winter Wyvern's Winter's Curse

Expected: All of them should be able to reduce/block Poison Sting/Dispersion/Envenomed Weapon damage. Its damage is only supposed to not cancel consumables and daggers, and not ignore damage reductions.

Note: Damage amplifiers like Soul Catcher, Bloodrage or Penitence, all work just fine, despite them usually ignoring hp removal as well.