How to delete a remote branch using magit in emacs

You can type y (magit-show-refs) to go to the refs buffer, and then k (magit-branch-delete) on any remote branch to delete it.

The built in branch delete feature of magit (magit-status, b k; or directly call the function magit-branch-delete) will delete remote branches in addition to local branches. Instead of specifying my-branch as the branch to delete, make sure to specify my-remote/my-branch.