pretty straightforward: have threads posted by blocked users be hidden or marked, similar to how forum posts by blocked users are hidden:
since threads usually involve more than just the original poster, it might be easier to replace the poster's username with "blocked user" or something similar instead of hiding the entire thread.
here's an example i made using MSpaint and one of my own threads:
There's the possibility that someone would change their username to "blocked user" or whatever replacement was chosen so perhaps add a color or italics to the text as well?
i feel like this would be most valuable in the trading boards but i'm sure it has other benefits.