How do I plug the hole that is in the back of the downspout when I do not wish to use the pop up assembly?

See the image below just remove the pop out and leave every thing in place would do the job, Thanks.

They make a chrome cock-hole covers that will cover the holes. Will not look good but will work. Good luck, Tom

Take a piece of plastic bag, put it around the hole and wrap with rubber band. Inexpensive, simple approach - but works

Milo