My policy is that discussion, no matter how vitriolic or out of order, ought to be preserved, even if for no other reason than that regular, established forum members participated in such a discussion and deserve to have the context of their replies preserved.
I sometimes leave the forum and come back to a discussion where some troll posts angrily and we respond. Okay. With the troll's posts preserved as what they are, I can follow the thread of conversation and know the reasons behind people's moderation decisions. Delete posts or threads and I come back confused and wondering why people are suddenly so angry at this invisible person.
So yeah, my arguments are mostly in the spirit of preserving information to avoid confusion. But I draw the line at commercial spam, pornographic or not. That is not a part of any discussion on these forums, and if people want to make humorous remarks in reply to it, then in lieu of deletion, said post can be edited into something funny to fit in. :3
Banhammering someone for being too much of a jerk is quite acceptable, though I will graciously leave things like total IP or email bans to the Reitz family for dealing with repeat offenders, as I otherwise believe in second chances if you are capable of taking a hint.
Also, all my policies are subject to change. I can reevaluate my position on matters, decide that I was wrong and that I need to behave this way now, and even change my mind back again later. That's what growing does to a person. I no longer have the sheer zeal I used to have.