I am assuming the two wires fed from the lower bus bar lugs feed the remote barn. Obviously turn off and verify that you have de-energized the wires in question before you attempt anything.
You have three/four options IMO:
:
A, Disconnect the two wires and remove them from the conduit all the way back to the sold barn, with new owners' approval, sell wire for scrap/beer money.
B. Cut the wires, leave in the box and safely tape/insulate, which will make the wires forever too short and unusable.
C. Cut the conduit and tape the wires at ground level, plug the conduit hole to keep out wasps and backfill the hole of the cut conduit and forget. (leave the cut wires varying distances longer above the cut conduit, after taping/insulating cut ends, bend wire 180 degrees and shove ends back into the conduit before burying)
D. You can disconnect all the wires in the conduit, cut the conduit but not the wires at ground level, tape insulate the wires safely, and bury the coiled wires in the ground, allowing a future reconnect without too much hassle/cost. I prefer this option the least, but feel it should be mentioned.

Anyone qualified addressing the other end at the barn in the future would normally remove the wires from the other end, disable or remove them to their satisfaction. Anyone not qualified might ignore and/or do nothing, at their own potential peril.
I will not address the legality of doing any of the above regarding local codes, but everything I have noted above I believe to be safe practice. It would be proper to tag the wires at the other ends as disconnected feeders with a date.