This is a very big question, does necrovalley stop the dark world from coming back?
I don't think so. necrovalley's rulings say that it doesn't negate cards that activate in the gaveyard. so it DOES NOT negate the effects of treeborn, vampire lord, twin headed behemoth, sinister serpent, sangan, mystic tomato. so if it would negate the dark world, it would negate those cards as well.

That is correct logic and you yourself have answered your own question.

Dark World monsters are not negated by Necrovalley.

Only things that can help prevent them...

The End of Anubis
Royal Oppression
Jowgen the Spiritualist
Banisher of the Light
Banisher of the Radiance
Macro Cosmos
Divine Wrath
Non Aggression Area
Ground Collapse and/or Ojama Trio and/or Ojama King to fill up their zones.

i thank you very much for your help. my girlfriend has a dark world/gravekeepers desk and at the shop we go to these judge wannabes try to say that they dont get the effects. THERE IS NO KNOWLEDGE THAT IS NOT POWER!!!!!!!!

Well, Necrovalley is still ranked as a very confusing card which still has some the most asked questions about it. Not to mention, judges are still human.