Previously,Stop service which controls virtual bridges.
sudo systemctl stop libvirtd.service #Systemd OS
Then,take down the bridge.
sudo ifconfig virbr0 down #centos6 centos7
Finally,remove this bridge.
sudo brctl delbr virbr0
Additionally. If libvirtd.service is restarted. the bridge will created automatically. So you better disable this service.
sudo systemctl disable libvirtd.service