The way to delete a pending task depends on whether it is planned or unplanned:
Unplanned task: if the task is unplanned, you can easily delete it by clicking on the trash icon located next to the task in the pending tasks list.
Planned task: if the task is planned, you will not be able to delete it directly from the pending tasks view. To delete it, you must go to the task plan where it was originally scheduled and delete it from there.
Note: When modifying a task plan, all pending tasks associated with that plan will also be modified.