Interesting, there is apparently no way to remove posts from google reader cache (even when the post is deleted from site/rss feed). Google reader will never ever remove deleted posts from its cache.

Good to know when you accidentally post to the wrong blog :) If that happens it is better to change the content of the post than delete it, then google reader will at least update it’s cache with the new content.

For more info:

http://superuser.com/questions/56908/remove-deleted-posts-from-central-google-reader-cache