bash

download https://rtyley.github.io/bfg-repo-cleaner/

clone `git clone --mirror git://example.com/some-repo.git`

if your files to delete are in the last "master" commit, delete the file and push 
.   
The last master commit is always preserved so you need to make another commit.

then `bfg --delete-files filename.txt  my-repo.git`
my-repo.git is the name in your folder

finally
`cd some-big-repo.git`

`git reflog expire --expire=now --all && git gc --prune=now --aggressive`

in the end

`git push`