UPDATE:

You can add other branches to exclude like master and dev if your workflow has those as a possible ancestor. Usually I branch off of a "sprint-start" tag and master, dev and qa are not ancestors.

First, list all branches that were merged in remote.

git branch --merged

You might see few branches you don't want to remove. we can add few arguments to skip important branches that we don't want to delete like master or a develop. The following command will skip master branch and anything that has dev in it.

git branch --merged| egrep -v "(^\*|master|dev)"

If you want to skip, you can add it to the egrep command like the following. The branch skip_branch_name will not be deleted.

git branch --merged| egrep -v "(^\*|master|dev|skip_branch_name)"

To delete all local branches that are already merged into the currently checked out branch:

git branch --merged | egrep -v "(^\*|master|dev)" | xargs git branch -d

You can see that master and dev are excluded in case they are an ancestor.

You can delete a merged local branch with:

git branch -d branchname

If it's not merged, use:

git branch -D branchname

To delete it from the remote use:

git push --delete origin branchname git push origin :branchname # for really old git

Once you delete the branch from the remote, you can prune to get rid of remote tracking branches with:

git remote prune origin

or prune individual remote tracking branches, as the other answer suggests, with: