You can only delete benefits while they are connected to a tier. If you are trying to clear clutter from your Benefits page, you will first need to add the benefit back to a tier in order to permanently remove the benefit from tracking.
To remove a benefit from a tier:
Go to your Creator Page Editor by clicking on the Page button in your menu, then selecting the Edit page link. Then, click on the Tiers tab link.
Select the Edit tier button at the upper right corner of the tier you want to remove the benefit from.
In the benefits section you'll find a list of currently tracked benefits for that tier. Click the X to remove a benefit.