How has OP and noone in comments mentioned the Combat Deployment Dispatch Mission? When you obtain too many demon points, a Dispatch Mission titled "Refugee Rescue" will appear in your list of available dispatch missions. It is a high cost mission at S+ tier, but it completely resets all demon points back to zero. White Mamba takes quite a while even if you skip the boss by one shotting him from afar. The easy way is to just farm online FOBs for good soldiers, you'll level up your base immensely as you go, and then spend some S+ on the dispatch mission to reset the demon points

one last thing demon points ARE NOT tied in any way to heroism points

oh also you can theoretically only reduce your demon points by 5160 by doing white mamba so it'll take a few times considering it's in the tens of thousands and there's of course the refugee mission as stated previously

Removing Demon Points
-30 - Fulton extract a common animal, puppet soldier, or Parasite Unit member.
-60 - Fulton extract a rare animal, Diamond Dogs staff member, or a prisoner.
-60 - Extract a puppet soldier, a critically wounded enemy, or Quiet via helicopter.
-120 - Fulton extract a very rare animal, an extremely rare animal, or a child soldier.
-120 - Extract a VIP, a prisoner, or a Diamond Dogs staff member via helicopter.
-240 - Extract a child soldier via helicopter.
-300 - Visit the Animal Conservation Platform.
-1,000 - Dispose of a nuclear weapon.
-1,000 - Earn the "Cry Havoc" or "Speechless" trophy.
-5,000 - Earn the "Cleared", "Trusty Steed", "Man's Best Friend", "War Buddy", or "Reminiscence" trophy.
-10,000 - Earn the "Conservation" trophy.
-30,000 - Dispose of ten nuclear weapons.

Earning Demon Points
+20 - Fail the Fulton extraction of an animal.
+20 - Kill an animal.
+30 - Critically wound an enemy soldier.
+40 - Kill an animal using fire damage.
+60 - Fail the Fulton extraction of an enemy soldier.
+60 - Kill an enemy soldier.
+60 - Lose a Diamond Dogs staff member held hostage by aborting mission 22 (+60 each).
+75 - Destroy D-Walker.
+90 - Kill a Diamond Dogs staff member held hostage.
+100 - Kill a prisoner.
+120 - Kill a critically wounded enemy soldier.
+120 - Kill an enemy soldier using fire damage.
+150 - Destroy a support helicopter.
+150 - Critically wound D-Dog, D-Horse or Quiet.
+180 - Kill a Diamond Dogs staff member.
+180 - Kill a Diamond Dogs staff member held hostage using fire damage.
+200 - Kill a prisoner using fire damage.
+50,000 - Develop a nuclear weapon.

sorry have to break this into 3 comments because of the character limit

here's everything i could find on demon points:

Normal: This is the default appearance for Snake and can be re-obtained by reducing Demon Points below 40,000.
Horned Snake: This form appears after obtaining 20,000 points, the first time. It is distinguished by the metal shard in Venom Snake's head growing significantly to resemble a horn. If the player returns to 'Normal' form after becoming Horned Snake, the player will return to Horned Snake after 40,000 Demon Points from then on.
Demon Snake: This form appears after obtaining 50,000 points, the first time. It is distinguished by the metal shard in Venom Snake's head growing significantly longer than even Horned Snake and by Snake becoming permanently covered in blood. If the player returns to 'normal form' after becoming Horned Snake, they will return to Horned Snake only after obtaining 80,000 Demon Points in total.

If memory serves the wiki states if you keep being a bad Snek and murderlizing people and your >metal< shrapnel grows fully erect, a combat deployment becomes available, I think it was.. refugee rescuing or something which takes a few days. When that finishes succesfully you literally get rid of demon form PERMANENTLY. Which might mean, you can never reattain it.

Definitely keeping this if I have a run-in

There is really no other way other than waiting for a week

I made a nuke and i removed it which increased alot of my demon potions like - 50000 heroism which all i can do to remove that is to keep playing mission 23 over and over