Similar to gist:942899, deletes all remote branches that have already been merged into master. It's just a lot more nervous about doing the deletion.

git_prune_remote.sh

#!/bin/bash
# -*- mode: shell-script -*-

EXPECTED_ARGS=1

if [ $# -ne ${EXPECTED_ARGS} ]
then
    echo "Usage: git_prune_remote <remote name>"
    exit 2
fi

REMOTE_NAME=$1

for branch in `git branch -r --merged | grep ${REMOTE_NAME} | grep -v master | xargs -L1 | awk '{split($0,a,"/"); print a[2]}'`
do
    while true
    do
        read -p "Delete merged remote branch ${branch} on ${REMOTE_NAME}? [yes|no] " yn

        case $yn in
            yes ) git push ${REMOTE_NAME} --delete ${branch}; break;;
            no ) break;;
            * ) echo "Please answer 'yes' or 'no'";;
        esac
    done
done