Link: Proxmox Forum

Automatically

1. Go to your server
2. Expand the disk section
3. Click on Directory
4. Find the directory you want to remove
5. Click on the obscure more button in your top right corner
6. Select Destroy

Manually

1. Login to your Proxmox Server Web GUI
2. Remove the Storage from the Server
   1. Go to your Data Center and Select Storage
   2. If the same storage name is being used elsewhere,
      1. Click on the storage name and edit.
      2. Remove the server name you want to disassociate the storage from
   3. If it's only used on one server,
      1. Click on the storage name and remove
3. Find the mounting service from your terminal and disable it.

systemctl list-units "*.mount"

4. Remove the mount file
   1. Look for the mount you're looking for in /etc/systemd/system. It'll look like this mnt-pve-[mount name].mount
   2. Delete that directory
5. Unmount the directory from the server

umount /mnt/pve/[folder name]

5. Remove the directory from the server

rm /mnt/pve/[folder name]