Quote - We already have the facility to add advisories to our posts ('language', 'nudity' etc.).

Why not two more: 'Off topic' and 'Warning'?

The 'off topic' advisory would normally be applied by the poster. It would allow for diverse discussion to happen within a thread, and yet make it easy for people to view the whole thread without those posts by modifying their forum profile to ignore off topic posts.

The 'Warning' advisory would normally only be applied by a moderator. It would serve as a public warning that the post was inappropriate in some way. Users can choose to not see posts flagged with a 'warning' advisory by modifying their forum profile.

This approach makes use of the forum functionality (advisories) that already exist and results in no deletions or censorship at all. It is left to each user to decide what they want to see.

 

This post made me think that, perhaps, forum interfaces will have to evolve at some point, and I don't mean only this one, I mean everywhere.  

Add more tools for both the staff and the users, for example, like snarly's idea of more flags, a moderator could flag a post (or user, just for the thread, if they keep it up) and then, others, could filter out certain flags, like, I walk in an interesting thread, see some inflamatory posts that are flaged, filter the "warning" flagged posts, and bingo, I have a clean thread to read peacefuly.