Check the vm config file via shell maybe the storage disk is still referenced there. If so delete the line and hopefully you can just delete is via GUI. The file path is /etc/pve/qemu-server/<VMID>. If that doesn't help removing the storage and vm config via ssh should remove it from the web gui but idk if you're leaving stuff behind if you do that. https://forum.proxmox.com/threads/how-to-force-remove-container.105341/