Cannot rollback addition of repository

[Roderick <hruodr@gmail.com>]

I wanted to do in a new debian stretch installation

apt-get install certbot python-certbot-apache

and had no success. Then I followed the instructions of

https://certbot.eff.org/lets-encrypt/debianstretch-apache and did

[1] apt-get install certbot python-certbot-apache -t stretch-backports

without success. Acording to

[2] https://backports.debian.org/Instructions

I added the line

[3] deb http://deb.debian.org/debian stretch-backports main

to the file /etc/apt/sources.list and did "apt-get update".

In contradicction to these Instructions in [2],

[4] apt-get install certbot python-certbot-apache

*did* install the packages, without the -t flag. I deinstalled them with

apt-get remove certbot python-certbot-apache
apt-get autoremove

Then I commented out the line [3] in /etc/apt/sources.list and did
again "apt-get update".

But: Command [4] still install the packages. I am not in the same
state as at the beginning.

Rodrigo