Considering my base is really high up (map height actually), someone actually made a stair from ground-level up to the map height. (see first image). At first he used wooden stairs (see second image), then later on he used wooden plates all the way up to map height (definitely took him lots of hours to do this). Then on the top, I don't know what happened (probably died due to lack of air), so what I did was I removed all the plates, stairs, beds that the griefer tried to use, then left a single plate as a proof. (See third image).
/position would have been most helpful. Don't remove plates in case there's more than one griefer in the party. I managed to find the plate. J Dog banned for the second time.