1.Stop Mongo DB using -

sudo service mongod stop

or for CentOS7

systemctl stop mongod

2.Remove Packages using -

sudo yum erase $(rpm -qa | grep mongodb-org)

3.Remove Data Directories using -

sudo rm -r /var/log/mongodb
sudo rm -r /var/lib/mongo

From: https://stackoverflow.com/questions/8766579/uninstalling-mongo