The answer from Sagar Rakshe is correct.
Here I explain how to perform it manually.
Remove the repository
First, identify your wrong file in
(you may also have to check the main file
Then, rename the file to disable it:
sudo mv WRONGFILE.list WRONGFILE.list.backup
Or instead of renaming the file, create a backup directory and move the file there:
sudo mkdir backup_on_date_YYYMMDD
sudo mv WRONGFILE.list backup_on_date_YYYMMDD
For your information,
add-apt-repository is a python script adding or commenting lines in your
*.list files. Personally I prefer to handle myself these operations in order to keep clear & secure
Remove the package signing keys
Show the installed apt keys with
apt-key list and find the key for the repo you want to remove, for example:
pub 2048R/5044912E 2010-02-11
uid Dropbox Automatic Signing Key <firstname.lastname@example.org>
You can then use
sudo apt-key del 5044912E to remove this key.