Task assignment email suppression

Customers are getting Task assignment email even though we have following setup in custom.properties file :

server.conf.notifications.process_new_task_me_timing=never
server.conf.notifications.process_new_task_others_timing=never
server.conf.notifications.process_new_task_unassigned_timing=never

Can someone please advise on what else we have to setup to suppress these automatically generated email.

Thanks!