How to properly close a chimney hole in the basement
Assuming the flue is not shared by any other devices of combustion on the first floor or above, you can use anything temporarily to block the draft and keep vermin out. you can use some mortar with a metal backing plate. Cut a piece of sheet metal in a rectangular shape slightly narrower than the hole diameter and a few inches taller than the hole. Put a long skinny bolt in the center to use as a handle.  Insert the metal and move it until it is flush against the back blocking the hole. You can then put a scrap of wood with a hole drilled in the center over the bolt and use a nut to secure the metal plate against the back of the hole in the flue.  This will stop your mortar from falling down into the flue and allow you to fill the void completely.  To make it look OK until you renovate, cover the repair with a thimble cover.  There is no need to remove the flue adapter ring.