Thank you once again for your reply Huisheng Xu :) ,
Do you mean that if one of forked execution being removed, the processInstance can not be removed?
I could not get you, as you created a fork, then there will be two processInstance and when User who applys want to cancel his application before being it get assigned to any other user by Admin, the two processInstance should get deleted.
If you use default configuration and do not change ends attribute of end activity to 'execution', the end activity will terminal the whole process instance.
I really could not get you, can you explain more on this also.