Is there a way to prune posts, as opposed to pruning threads?

We have a number of long-running threads from which we would like to delete old posts.

The only functionality I find is to prune entire threads, but not old posts within the threads.

Is this funtionality available?

From the topic page, click Admin -> Manage. Under replies you can tick the box next to each message and then hit delete. You have to do this per topic though. I don't know of any way to mass prune posts (as opposed to topics).