To disable Task EDA events globally (for all task types), go to Administration → Business → Tasks → EDA Events Configuration, set “Produce EDA Events” to Disabled and save. Effect is immediate, no need to restart.
To disable Task EDA events for a specific task type only, select the Task Type in Administration → Business → Tasks → Task Engine Administration, go to the EDA tab, uncheck the 3 Emit Event checkboxes (Task Created, Task Updated, and Task Deleted), and click the Update button. Effect is immediate.
However if as you say the task does not exist, you may be seeing event re-delivery attempts by Nerv.
If that is the case, options 2 or 3 suggested by Atanas should help.