To delete a task, you should go to the GUI (Maintenance->Snapshot) then press on the plus sign next to the volume group name, and click on the snapshot.

At the end of the page "Schedules for snapshot task" you will see list of all tasks and next to each task there will be a red "X" for deleting the task.

Also find this forum post, it might help:
http://forum.open-e.com/showthread.php?t=2634