That's correct. The FE adapter senses the carrier from remote party, brings the link up but no frames are processed on this FE interface.
There is no JUNOS command I know of that will bring link down on copper FE interface. Try to disable the remote party FE interface if it is not a JNPR box.
If your goal is to bring link down _whatever_ it takes, try to offline the whole FE PIC. Unfortunately, individual FE ports cannot be offlined.
Rgds
Alex