git - Deleting remote master branch, refused due to being current branch - Stack Overflow

In the settings block on the options tap in the settings page (well, just click on the Settings tab on your github repo page), you are able to change the default branch (you need to have the other default branch on github as well).

After you have done that, you are able to remove it:

$ git push bb :master

Deleting remote master branch, refused due to being current branch