When I was an admin on another forum we had some performance issues (it was popular) so it was decided to purge the older threads. I was rather enthusiastic.
I got a reputation and had to stop - and it wasn't helping anyway.
What we did do was create a hidden area called "the workroom" where we could move threads that seemed to have gone off the rails. We could then decide to "sorry, we over reacted" and move back, purge posts that caused all the grief and move back or simply deep six it, never to see the light of day again. Having this special spot acted as a built in "undo" button and has saved some bums over the years. Highly recommended.