Proxmox Reset Cluster

First, make a backup of the cluster:

cp -a /etc/pve /root/pve_backup

Stop cluster service:

/etc/init.d/pve-cluster stop

Umount /etc/pve if it is mounted:

umount /etc/pve

Stop corosync service:

/etc/init.d/cman stop

Remove cluster configuration:

rm /etc/cluster/cluster.conf

rm -rf /var/lib/pve-cluster/*

Start again cluster service:

/etc/init.d/pve-cluster start

Now, you can create new cluster:

pvecm create newcluster 

Restore cluster and virtual machines configuration from the backup:
cp /root/pve_backup/*.cfg /etc/pve/
cp /root/pve_backup/qemu-server/*.conf /etc/pve/qemu-server/
cp /root/pve_backup/openvz/* /etc/pve/openvz/
UPDATE: This post is also valid to change the hostname of a node in a cluster or to move a node between two clusters. When you have removed a node from the cluster, it still appears in the proxmox nodes tree, to remove it from the tree you have to delete the node directory from another node in the cluster:
 rm -rf /etc/pve/nodes/HOSTNAME