CANCEL_UC_OBJECT or DEACTIVATE_UC_OBJECT will work. Just need to get the runid of the RemoteTaskManager as it is kicked off. Simple enough with the GET_UC_OBJECT_NR.
The following would be kicked off in the Child Post Process tab.
: SET &RC# = GET_UC_OBJECT_NR(JOBQ.PS.PROCESSREQUEST.NEW.1)
: SET &STATUS# = CANCEL_UC_OBJECT(&RC#,"ENDED_CANCEL")
Can also use ENDED_OK and ENDED_TIMEOUT for the 2nd parameter.