Not able to delete set Value control


I am using map control to populate value in one document reference variable. Few of the sub variable of this document reference are hardcoded by using set value control. Now i want to delete few set value control from this document reference but i am not able to do so.

Where as i am able to delete set value from few string variables which are not part of document reference variable.

Can you tell me why IS is not allowing me to delete these set value control.

I am using WM IS 6.1