I went out and read some "mainstream" explanations, and man, there are some varied (and wild) reasons given that a) this isn't talking about people but demons b) these were imprisoned demons from the fall and the flood reference is just "an example of God's patience", c) Jesus didn't evangelize, he proclaimed, letting those demons in that underworld know He had "conquered", d) the worn out phrase over and over, "there is no second chance after death".
I wanted to get those "out of the way", as we delve into Shawn's question of spiritual death rather than physical death. Another factor for me is the last part of the verse. I'll have to dwell on this some more, good question.