Delete a remote branch

git push origin :<BRANCH>

Delete local tracking branches

git fetch -p origin

Delete untracked files

git clean -fn

Remove from repository all locally deleted files

git rm $(git ls-files --deleted)

Delete all untracked files

git clean -f

Including directories

git clean -n -f -d

Delete files from repo already deleted from disk

git ls-files --deleted -z | xargs -0 git rm