Currently, the only supported method to stop background processes is to stop and start the Tableau server. This is a good one to put in the Ideas section!
I too have the same issue. But even after stop & starting the Tableau Server, then also the back ground tasks are running.
that is, the tasks that were running during the STOP were cancelled. but the task which were waiting state have started after START
please let me know if any way to kill all the running tasks.
disable the associated schedules before the restart. They will not kick off when you come back up unless you reenable.