I've been having this same issue. Hector from the GLL staff deactivated my registration so I could try installing and joining the course again, but now I can't proceed because I already have the /github-slideshow repository on my account. Is there any way I can delete this repository and try again?:
Sure @jillonymous, just navigate to the repo by clicking on the title in the red bar that appears, then go to the Settings tab, scroll all the way to the bottom and click the button to delete the repo. Type in the full repo name (including your username in front of it) and that will delete your repo. You should then be able to re-register for the course. Full instructions for how to delete your repo are also available on the GitHub help docs.