If it's a concentration spell, you can cancel it yourself anytime, select the character that is concentrating on it, and down under the portrait on the left there will be a little icon with an x, hit the x to cancel concentration.
Some acid and poison aoe effects don't actually cause direct damage, they will just inflict you with a status, but the game seems to have difficulty managing npc pathing for things like this. I've had companions refuse to go into a non-damaging poison cloud, but then walk into fire on the ground and take damage.
