Hi Anand,
If you have a look at the source of EndActivity, you will find out the default behaviour of EndActivity will terminate the whole processInstance. So the forked executions of process instance will be deleted cascade.
I think you could try what Maciej said, using variable to decide whether starter could cancel the process instance.
At this moment, I am studying BPEL, the different between BPEL and jPDL is jPDL hasn't a scope for activities, so we will find it is hard to do something should be done in a scope by jPDL. Maybe we could use sub-process to emulate a scope.