You can NOT cover it if there is ANY power to the box.So if there is electricity going to it, and the electricity is used to feed other boxes, it can't be done.
The problem here is, a hidden (once it's covered by plaster) box COULD then have power to it and it WOULD be against code.But if the electricity can be shut off at the breaker box, do the wires have to be moved out of the breaker box?
Go back and reread what they posted. They stated that the wiring to the box is dead. As long as it cannot be reattached at either end, I personally do not see a issue with covering it over. I though would pull the box out, so that it can just be covered, since there is not a need for it anymore.You CAN NOT plaster over or cover any junction box that is active. Junction boxes need to be readily accessible.
If the box is disconnected, remove it, and then patch the hole.
The OP asked "Is it OK, if junction box has electricity disconnected, or the box is covered, to plaster over the box?"Go back and reread what they posted. They stated that the wiring to the box is dead.