I use this PowerShell variant:

    function Remove-GitBranches {
        git branch --merged | Out-GridView -Title "Branches to Remove?" -OutputMode Multiple | % { git branch -d $_.Trim() }
    }

`Out-GridView` gives you a quick popup dialog with all the branch names that supports easy multi-select. That way you get a quick preview of what you are cleaning up and can skip work in progress branch names that you haven't committed anything to yet.