Users on the Forever-free plan have access to one watch list which can not be deleted. To edit this watch list, including renaming the list and removing apps follow this article: Editing a Watch List
Users on the Pro-version and the Enterprise plans can delete a watch list by following these steps.
Step 1 - Open a Watch List for editing
By clicking the chevron at the top of the page, watch list selection is expanded and the ability to edit your watch lists revealed.
Select the watch list you would like to delete by clicking the pencil icon:
Step 2 - Delete the Watch List
Clicking Delete will bring up a confirmation:
Click Delete to remove the watch list. Click Cancel to go back a step without deleting.