Is the task a workflow task? Sometimes overdue task notification are kept sending even when the task is set to “Completed”. The cause of this problem is entries stuck in the ScheduledWorkItems table. The solution is to write some codes to use OM to delete the broken workflows. However it is not supported to modify SQL database and this needs development knowledge. If you want to ask for help about the programming problem, you can post your question in the development and programming queue.