Pruning objects from history?

I've imported the full history of a large project from Subversion using the latest git-svn. The resulting repo is huge, and I believe it's due in large part to a series of big tar.gz files that got checked into the Subversion repository by mistake early in the project's history. They were subsequently removed from svn, but of course git-svn grabs them and puts them in my local history.

Is there any way to excise those files? They are of no interest to us now -- they were data files for a third-party application we ended up not using -- and they're making git look bad in the disk usage department.

I believe this has been asked before in the context of removing copyrighted content from public repositories. However, I have a twist that may make it easier: nobody else has cloned this repository yet. I am free to rewrite history with no risk of messing up any downstream repositories, and I don't have to worry about propagating the deletions out to anyone. I just don't know how to do it (assuming it's doable at all.)

Thanks!

-Steve