Just look for the repository entry and delete it from the file.
sudo nano /etc/apt/sources.list
Optionally one can remove repository using add-apt-repository command
sudo add-apt-repository -r ppa:PPA_REPOSITORY_NAME/PPA
To delete repository key, fist list them and fine the problematic one.
sudo apt-key list
This command will list all added repository keys
/etc/apt/trusted.gpg pub rsa4096 2015-07-25 [SC] [expired: 2019-07-25] EDB7 D030 4E2F CAF6 29DF 1163 0757 21F6 A224 060A uid [ expired] openHAB Bintray Repositories firstname.lastname@example.org /etc/apt/trusted.gpg.d/ubuntu-keyring-2012-archive.gpg pub rsa4096 2012-05-11 [SC] 790B C727 7767 219C 42C8 6F93 3B4F E6AC C0B2 1F32 uid [ unknown] Ubuntu Archive Automatic Signing Key (2012) email@example.com
In this case to remove one related to “openHAB Bintray Repositories” (specify at least the last 8 characters of the key)
sudo apt-key del A224 060A
Tip: In may case the problematic repository was in /etc/apt/sources.list.d/ openhab2.list , so I removed the whole file.