I have setup auto prune for me trash bin (aka basket) yet it is not working. Is it just an area that you cannot use auto prune? http://2girlsgraphics.forumotion.com/
The auto-prune is the feature that sends the topics to the forum basket after it reaches a certain amount of posts, so I don't see why it would function in your forum's basket. I don't use it myself but it would make sense.