Can an octagon fixture box that was also found to be a junction box, be closed and covered over in the ceiling after remodeling. Box not needed.
Absolutely not. Any box containing splices or live wires, even if it's not being used, must remain accessible.
As long as there are no cables in it, you can rip it out or cover it. If there are cables in it, you can't.
You state that the "box is not needed"; I presume you mean not needed for the fixture but that the wire connections originally present still remain. If that is the case then you cannot bury it. An alternative, if there is access to the box from above via a crawlspace or attic, would be to replace or re-orient the box so that access is provided from above. Then you could cover the hole in the ceiling.
Depending on your local code, you may be able to use one of these new type splice units, and eliminate the junction box. These are available at home centers and electrical supply houses, one brand is Tyco. Ask for a in-wall romex splice kit.
Listen to the first answer it is never permissible to cover(hide) a junction box behind anything. God Bless Big Jim