First of all, Soulfire is not mortal magic so it is technically not subject to the Laws. Secondly, Soulfire can do ANY thaumaturgy at the speed and methods of Evocation. So instead of the spell being Evocation - and thus having to be block, maneuver, attack or dispel - it can be Thaumaturgy and thus any type of effect you want. This gives us a few more options;
1) A 7-shift Ward vs agressive actions applied to the entire Zone. Any hostile actions have to contend with a block of 7 that does not break even if momentarily overcome and reflects attacks back on the attacker if the attacks don't overcome it. I think attackers are going to get the idea pretty fast.
2) A 7-shift zonewide veil vs agressive actions. Yes, it is a neuromancy spell. No, it does not affect the mind. It affects the sensory organs directly each time someone tries to attack, applying an illusion of sensory deprivation for all senses (including balance). You can't attack what you can't see, hear, smell or otherwise locate.
3) A 7-shift zonewide biomancy block vs agressive actions. It works by making you utterly calm and non-agressive, similarly to being chemically sedated only more benign and without the side-effects.
4) A 4-shift conjuration for mass, 1 shift conjuration for complexity, 4 shifts to apply the aspect "transparent". What does this do? It conjures a 3-ft-thick steel bubble around every person in the zone that happens to be transparent. Go ahead; try to shoot through 3 ft of steel. You can still talk and make funny faces through it though.
5) 7 shift zonewide block vs ignition. Sorry, firearms and explosives no longer work anymore. Feel free to use them as short clubs now.