User blog comment:ImperiexSeed/Michael or Death, who's stronger?/@comment-92.23.252.19-20120428191313

Death's limits:

Death couldn't erase Sam's Hell memories

Death can't destroy souls

Death can be bound and locked up

Angels can erase some memories so it's possible the Archangels could fix Sam, Archangels can kill Demons (which seems to be destroying souls) and, because Death didn't destroy Chicago, the binding spell doesn't seem to be that effective.

Because of this, I don't think Death would willingly lock himself up and so I think he was locked up by a stronger being. Death was locked up after the Flood (it was after Humans were created) and by that point God, Lucifer and Gabriel had probably already left Heaven so it was probably someone else. Raphael was weaker than Soulstiel and as a result is weaker than Death. That only leaves Michael.