I second the recommendation to use git rocket filter [1] from this link.
A guy in our team committed a big file unnecessarily into the repo which already had a year's history behind it, and it was only a month later when I found out. git rocket filter filtered it in a few seconds.
Since all team members had already gotten it, I ran it on all their repos to verify it's gone, and ran the usual git gc incantation [0] to clean their repo.
