removing content from git history

I assume that this question has already been addressed on the mailing list, but I wasn't able to find anything about it in the archives.

Is it possible to remove content entirely from git's history? I have a client who does not use git for version control. A couple months ago they committed some sensitive client information which should never have been committed. Recently, they realized the mistake and now want to remove all traces of the mistake from history.

I would like to migrate them to git at some point. However, if they had been using git for version control already, I'm not sure how I would solved this particular problem. Any suggestions?