I have:

• title: Iron Protection Stone (15x15x15)
  block: 42
  radius: 7
  custom-height: 512
  prevent-place: true
  prevent-destroy: true
  prevent-fire: true
  prevent-creeper-explosions: true
  prevent-creeper-explosion: true
  no-conflict: false
  price: 0
  limits:
  • 2
  • 3
  • 4
  • 4

In my config, but when someone (not /ps allow'ed) places TNT next to the field, the TNT still explodes and destroys blocks inside the field.

Also minor typo http://wiki.sacredlabyrinth.net/index.php/Field_Flag_Reference#prevent-tnt-explosions says prevent-tnt-explosions prevents creeper explosions :)

I don't see you using the prevent-tnt-explosions flag on that field you pasted. People will still be able break stuff with tnt. You will ant to use "prevent-explosions" or "prevent-tnt-explosions" flags. Fixed the typo, thanks.