Hi.
Can a NM clamp be in a distribution box knock-out hole without any cable trough it ; or needs to be removed and replaced by a blind cover ?

 

Call a proffesional Electrician before You create a dangerous situation and regret it.
At least You took pictures, I have to give You that.
.
.
.

 

Yes, it can be installed. BUT it will not stop a determined fool from gaining slight access to the box and causing problems.
The "level of danger" is what is the big concern. I have seen boxes with vacant openings exist safely for many years. But if there are those folks bent on causing problems, they would succeed if one was open.
Do you simply need to block an opening? or is this to allow for a future circuit?? A very short section of cable installed into the clamp would solve the problem with a minimum effort or cost.