How come buster blader the dragon destroyer swordsman with DNA surgery set to dragons. Didn't stop destiny hero decider's effect
I suppose you mean the effect Decider has that activates on the hand (to reduce effect damage inflicted to 0). Your Buster Blader couldnt' stop it because DNA surgery only affects monsters on the field, from it's text:
Activate this card by declaring 1 Monster Type. All face-up monsters on the field become that Type.
Because of that is that Destiny Hero - Decider was still able to activate its effect, because when in the hand it is still a Warrior and not a Dragon.
Seems that you were asking about the effect that states "Level 6 or higher monsters your opponent controls cannot target this card for attacks."
From Buster Blader's card text, emphasis mine:
[...] also Dragon-Type monsters in your opponent's possession cannot activate their effects.
However, that effect Destiny Hero - Decider you ask about is a Continuous effect. Those kind of effects do not "activate", as they are always and continuously taking effect. Thus, Destiny Hero - Decider does not activate that effect, this why Buster Blader couldn't prevent it from being "activated".