To completely remove a branch from your git server (including history), simply use the following command:
git push origin --delete [branch name]
Then you need to run the following command on the instances to remove the information of the deleted branch:
git fetch --all --prune
If for one reason or another, you need to delete one or more files from previous commits, here is an interesting article on the subject:
https://help.github.com/articles/remove-sensitive-data/