Remove remote braches that are merged into master

git branch -r --merged master | grep -v 'master$' | sed -e 's/origin\///' | tee | xargs -I branch -p git push origin :branch

Finds branches that have been merged into master, excludes master, replaces 'origin/' prefix, then deletes them all.

Thanks to -p option, console will always ask you for confirmation, so you do not delete branch by accident. Although it is still dangerous, so use this command with caution.

Based on @catchamonkey protip https://coderwall.com/p/oik3ta