Don't let them hit level six? Your own ships also become invulnerable if you stay close to it, so you shouldn't be getting anything picked off. Once it wears off you should destroy the Akkan ASAP.
Now three Akkans can keep armistice going forever potentially (at least they used to, I thought this was fixed in Rebellion). In this case you should retreat most of your fleet, only leaves what you need to destroy one Akkan. The AI will only use Armistice if it thinks it will lose, so using a smaller fleet can be a good answer. Once you destroy one or disable the abilities of them all (I.e. with a Trio of Kortuls), send your main fleet back in and finish them off.