Say your client has a long term site that used to rank at or near the top in Google. Over the years he employed numerous different SEO firms and is now stuck with tons of backlinks that Google now disapproves of. One of the people who built most of those is now serving in the Philippine army and cannot be reached. The other one just won the lottery and is not responding to Skype or e-mail.
My response would be for him to leave up the old site, build a new site and forward the old one to the new one in the hope that good link juice would flow through and bad link juice would not.
What do you think of this solution? Any other ideas? What about a case where the client is still ranking well for half their main key words?