My research on the review spam filter issue seems to indicate that a trusted reviewer can leave an Excellent review for an honest business but not for one google have down-ranked for their spam. Where an Exscellent one is deleted from search and the reviewers Local - Your Places - # Reviews list.
Certainly the businesses where Excellent reviews cannot be left have recent reviews deleted from their review history as well.
The only thing it is hard to determine is whether old reviews are removed for review spam alone or web content spam as well. However all the posters in the long thread mentioned have web spam content, in Places, in the search results Rich Snippets 'fed' to google or in the website, more keyword stuffing.
I believe the only way to recover is to treat the entry as Suspended and Delete it, probably a merged +Local Business page, and have the Places entry, website, blog and web directory entries de-spammed, and then start again in Places.
I have seen one person with (just?) review spam recover after a Delete and re-enter the ability to get reviews again.
