Proxmox Delete Cluster
Theres no real way to delete a cluster in the gui with out doing a fresh install (that i know of) but the following set of commands should stop the cluster service, force it to local, delete the configs and restart it like new.
systemctl stop pve-cluster
systemctl stop corosync
pmxcfs -l
rm /etc/pve/corosync.conf
rm -r /etc/corosync/*
killall pmxcfs
systemctl start pve-cluster