I copied this gist from somewhere, but forgot where, so I am putting it here

clean-git.sh

#!/usr/bin/env bash

# This has to be run from master
git checkout master

# Update our list of remotes
git fetch
git remote prune origin

# Remove local fully merged branches
git branch --merged master | grep -v 'master$' | xargs git branch -d

# Show remote fully merged branches
echo "The following remote branches are fully merged and will be removed:"
git branch -r --merged master | sed 's/ *origin\///' | grep -v 'master$'

read -p "Continue (y/n)? "
if [ "$REPLY" == "y" ]
then
   # Remove remote fully merged branches
   git branch -r --merged master | sed 's/ *origin\///' \
             | grep -v 'master$' | xargs -I% git push origin :%
   echo "Done!"
   say "Obsolete branches are removed"
fi

echo "Please, tell your teammates to run 'git remote prune origin' command in order to clean their local repository"