I vote for some prevention. It should be made clear to people in the user agreement that unless there is a really compelling reason to do so, we will not delete threads with discussion already attached to them. People need to decide in advance what they will post and then live with it.
Extraordinary circumstances, to me, include legal situations, threats, or something unsafe (like if someone posted their physical address).
Otherwise, people need to take responsibility for their actions before they make them.
I think a case by case basis is the best way to go.
My 2 cents.
