Stop Threads in Unity3d using C#

I am doing an application in Unity 3d and integration with iPhone, i am doing some Image recognition task in unity and when user hit sharing button it loads iPhone native task, like load actionsheet and uialertview. When i present some model view controller then my app crash in iphone, i am doing this on main thread. I don't know where i am doing wrong, but i think if i'll stop some threads in unity it may help, or if you guys have any suggestion please help me. This will be great for me. Thanks in advance.