git remote prune origin
Removes tracking for branches that are no longer on remote. For this case remote is origin.
git branch --merged
Shows you a list of branches that have already beed merged to the current branch. These branches can be considered candidates for deletion.
git branch --delete <branch_name>
Deletes the local copy of the branch.
`git push origin --delete <branch_name>'
Deletes the remote copy of the branch. For this case, remote is origin.