We are unable to find a way to modify an existing Task Persistence DB Cleanup job or delete it.
It looks like there is no task for doing that from IDM User Console.
In the tasks window there should be an option within Cleanup Submitted Tasks to modify an existing job:
If you want to delete a scheduled job you can use Delete Recurring Tasks, also available under the Task list.
If the above tasks and options do not appear in your list of tasks you'll need to login as a system manager and add the appropriate task and/or admin role rules to provide access.