Execute the following commands via proxmox shell or ssh terminal(recommened). In this way you don’t have to empty all VMs from a node before removing a cluster from it.

# sudo systemctl restart pve-cluster
# sudo pmxcfs -l
[main] notice: forcing local mode (although corosync.conf exists)
# sudo rm -f /etc/pve/cluster.conf /etc/pve/corosync.conf
# sudo rm -f /var/lib/pve-cluster/corosync.authkey
# sudo systemctl stop pve-cluster
# sudo rm /var/lib/pve-cluster/.pmxcfs.lockfile
# sudo systemctl restart pve-cluster
# sudo systemctl restart pvedaemon
# sudo systemctl restart pveproxy
# sudo systemctl restart pvestatd