Of the various plans I have considered, here's a breakdown of them.

Necron Pylons
Pros: known to work, technological capability already exists somewhere in the 'verse
Cons: have to rebuild most of the organic people from ground up (capability to make warp-independent biological sentients does not currently exist)
Requires large scale intervention

Ascension
Pros: immediate implementation possible
Cons: Ascension is suspicious and the Culture dislikes it

Tyranid Shadow / Ork Psychic field / Necron warp disruption field
Pros: Non-permanent warp disruption that greatly increases resistance if not confers outright immunity
Cons: Technological capability does not yet exist
Requires large scale intervention

Friendly Warp God
Pros: Best solution to calm the warp
Cons: Difficult, may not be possible,
Technological capability does not exist
Culture warp god may not be friendly; 'friendly' races' gods may not be friendly

Tau genetic resistance
Pros: Should be low impact intervention
Cons: Culture citizens applicable only
Applying it to other races require their accepting of mass genetic engineering and large scale intervention
Not a complete solution