I use this alias:

    prune-local = "!git fetch -p && for branch in $(git branch -vv | awk '/: gone]/{if ($1!=\"\*\") print $1}'); do git branch -d $branch; done"

1. Fetch the latest from my remote, removing any remote tracking branches that no longer exist

2. Enumerate local branches, selecting each that has been marked as no longer having a remote version (ignoring the current branch)

3. Delete the local branch safely