After our spring convention there were a lot of blog posts about getting ready for the convention. These were no longer pertinent. There was no way to get them off the dashboard (old dashboard) other than deleting or unpublishing each post.
A way of setting a dashboard expiration date for posts would help. Posts would not appear on the dashboard after the dashboard expiration date, although they could still show in the user's blog. It would be a selection for each post.
A second selection for a post would be post deletion date. After this date housecleaning would simply delete the post.
Another alternative would be to allow only posts newer than some number of days to appear on the dashboard blog. This would be a dashboard setting.