You should be able to delete an out of stock item directly from the Watch List.
If you are using a desktop browser, go to your Watch List or Summary page, select the checkbox to the left of the item on the list, and then use the delete button at the top of the page.
If you are using the eBay app, from Watching, hit the "edit" menu to reveal checkboxes, select the box by the item, then tap the trash icon at the top of the page.
