This Knowledgebase article provides information about removing a task during the execution of the task.
On the Application -> Task screen of the Neverfail Heartbeat Management Client, when a task is executed, for example by clicking the Run Now button, the listed task can be removed from the 'Task' list but will continue to execute. This is considered normal behavior. The task will continue to execute until it either completes or times-out, at which point the task will be deleted.
Neverfail Heartbeat V5.3.0 and Later