The solution is to remove Medibuntu from your list of repositories:
* Open a terminal
* Type gksu gedit /etc/apt/sources.list to edit the list of repositories (replace “gedit” with “pluma” if you are running MATE, or with “kate” if you are running KDE)
* Remove any line which contains “medibuntu” in it and save the file
* Type apt update in the terminal
Why do all of these instructions start with "Open a terminal"?
There are two ways of getting at this:
* Go to System > Preferences > Software Sources
* Go to the Other Software
* Uncheck anything containing the word "medibuntu"
* Enter your super secret password
* Click the Close
That's it. You're done.
You can also access Software Sources in Synaptic by going to Settings > Repositories.
Either way, your system will stop trying to access the Medibuntu repositories.