[git] I can't delete a remote master branch on git

I need to delete a master branch, but it's proving to be hard. I just want to clean that branch out and start new. I am deleting from the dev branch. I want master on GitHub to be clean.

 # git push origin --delete master

> To https://github.com/mymasterb.git  ! [remote rejected] master
> (deletion of the current branch prohibited) error: failed to push some
> refs to 'https://github.com/mymaster.git'

How do I quite simply start my master with a fresh slate?

To answer the question literally (since GitHub is not in the question title), also be aware of this post over on superuser. EDIT: Answer copied here in relevant part, slightly modified for clarity in square brackets:

You're getting rejected because you're trying to delete the branch that your origin has currently "checked out".

If you have direct access to the repo, you can just open up a shell [in the bare repo] directory and use good old git branch to see what branch origin is currently on. To change it to another branch, you have to use git symbolic-ref HEAD refs/heads/another-branch.

The quickest way is to switch default branch from master to another and you can remove master branch from the web interface.