Made a note about it 4 years ago, but it looks like the note is still pretty usefull. So one more time for en readers.

To remove all merged branches(except current -v ‘*’):

git branch --merged | grep -v '*' | xargs git branch -D

also I made such command for repo complete clean up:

alias git-clean="git branch  | grep -v '*' | grep -v 'master' | xargs git branch -D  && git reset --hard && git clean -d -x -f"