To delete a task, simply right-click the task you want to delete, then click Delete from the drop-down menu.
Deleting a task in the Month view
In the Month view, you can delete one or multiple tasks at the same time.
Tick the checkboxes corresponding to the task you want to delete, then click on the Month toolbar.
Click Delete in the confirmation message to accept your deletion.
For shared or group calendars, you can only delete their tasks if you have the Edit permission on these calendars.