Deleting a preplan can be done as follows.

When viewing a preplan, scroll all the way to the very bottom of the page.

You will see the DANGER ZONE area... Proceed with CAUTION!

If you just created a preplan and realized you don't need it, the dialog is fairly benign.

You will be asked to CONFIRM your decision:

However, for a preplan that has content, the dialog is purposefully attempting to be ALARMING:

As is the confirmation dialog: