I've never heard of yahoo banning sites... but I think they should and that's one of the reasons why they're behind google.
I had a site that was banned from google, but on yahoo it just seem to be limited to one listing for the domain name which only appeared when you specifically searched for it by domain name. Maybe that is what they do when they ban a site?
Nope I've never gotten de-indexed by any search engine before. Yahoo is pretty relaxed most of the time so you really need to put something inappropriate up to get deindexed completely.
i had a site that was listed on the first page of yahoo then suddenly it was gone for around a month, now its back but a few pages back from where it was. honestly cant work out yahoo, there trippers, the search results are a complete mess, no offense.
pretty rare to be totally de-indexed in yahoo although sometimes they leave one page and its pretty much the same thing