How do I delete old yum repositories and configuration files under RHEL 5 or CentOS Linux server?

You can delete old yum repositories and its configuration files from /etc/yum.repos.d/ directory

Method #1: Delete Repo Configuration Files Using the rm Command

Type the following commands
# cd /etc/yum.repos.d/
# ls -l
Sample outputs:

-rw-r--r-- 1 root root  954 Aug 11  2010 epel.repo
-rw-r--r-- 1 root root 1054 Aug 11  2010 epel-testing.repo
-rw-r--r-- 1 root root  561 Dec 14  2010 rhel-debuginfo.repo
-rw-r--r-- 1 root root  222 Dec 14  2010 rhel-source.repo
-rw-r--r-- 1 root root  235 Jan 16 15:27 srpm.repo

To delete epel.repo, enter:
# rm epel.repo epel-testing.repo
### ***************************** ###
### make sure you delete GPG keys also ###
### ***************************** ###
# rm /etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL
# yum clean all

Method #2: Delete Repo .rpm Package

First, find out repo package name, enter:
# rpm -qa | grep epel
Sample outputs:

epel-release-5-4

# yum remove epel-release-5-4
# yum clean all