Way too many number variables for either a style RVM or a SQL text replace in the post table.
Your options are;
Accept that the horse has bolted and leave it as it is.
Make your classifieds forums viewable to registered members only. It would take quite some time, if ever, for the search engines to "forget" those posts though...
Implement a contact number spoiler BBcode and use
MarkF's BBcode permissions mod to make it unviewable to guests. Your members can then choose to use the BBcode or not. Of course this would not be retroactive unless you decide to manually edit the 179414+ classifieds threads.