How does a site get blocked on Yahoo search?

Can a site be unblocked?

[peterdouglas]

This is something new to me. A client of mine has had a site up for about seven years. A little over a year ago, I redesigned it in CSS. All was going well until yesterday he called me up to say his site is blocked in Yahoo searches.

How can this happen? He thinks a competitor blocked his site (all the competitors are still coming up ok in the Yahoo SERP). Can it be really this easy for someone to block a site?

There doesn't seem to be much information on blocked sites out there. I found an article from last month here on webmasterworld but it still didn't clarify things.