Commonly, the "Stop VM" button in the Proxmox webinterface does NOT work for some reason. Here is how to stop a VM:

Every Proxmox VM has a numeric ID, here marked in red:

Let's say we want to stop the VM-101. Remember the ID, then open the shell / console of the proxmox node, not the shell / console of the VM you are trying to stop. Then type:

  # qm stop 101