Hi Daniel,
Why do you want to end a sub process, and don't want to main process go on running?
If you do that, the main process will go to a dead wait state, it won't go anywhere unless somebody execute signalExecutionById() on it. It will be potential risk on this scenario.
If you really want to achieve a feature like that, I suggest you using CustomActivity to implement your own activityhaviour. Then you could do whatever you want to do.
Regards.