If country is in your sphere of influence, it won't prevent you from declaring war on it. It will only prevent other countries from doing it(and prevent from some diplomatic actions with this country), if they have truce with you. It won't prevent them from this action if they DOES NOT HAVE TRUCE with you, but if they do it will give you causus belli. Having a country in sphere of influence have NO negative effect, except that every such country increase cost of bringing more countries to your sphere of influence. Don't answering call of country in your sphere of influence, that is not your ally or vassal, do not bring loss of prestige, or anything else, but if this country is an OPM, it might get annexed, and so you will lose a country in sphere of influence.
You can only lose a country in sphere of influence if:
It will get annexed
It will become too powerfull/too big/too strong economicaly in comapare to you - i am not sure if prestige count. I think not.
Country with bigger prestige add it into their sphere of influence
They will become a vassal of third country.
You will renounce them from your sphere of influence in peace deal.