1 - Stop/Out all OSDs
2 - Remove all OSDs
3 - Remove ALL Mons (except the master)
4 - Remove ALL Managers (except the master)
5 - Execute on each OSD node: pveceph purge
6 - On last node (master mon/mgr): stop all ceph services, and execute: pveceph purge
If you wanna run CEPH again, you need to remove all conf files on /etc/ceph/ and /var/lib/ceph first...
You can use the Webinterface for most of this. Just go to a nodes ceph tab and destroy first all OSDs, then all Managers and Monitors but not the last one.
On the last one just stop those two, then go to the commandline and execute pveceph purge to remove the ceph packages for each node.