I've had this in my ~/.bash_aliases for awhile:

  alias git-wipe-merged-branches='git branch --merged | grep -v \* | xargs git branch -D'

Trying to remember where I got that one, as I had commented the following version out:

  alias git-wipe-all-branches='git for-each-ref --format '%(refname:short)' refs/heads | grep -v master | xargs git branch -D'