how do I remove a registered account?

Hi! The title is pretty much self-explanatory, but here is the story to it:

I know I've been messing around trying to find the best username for this website, so I registered accounts left and right, but now I regret it. I've recently found the username I desire, ZREURNO, but when I try to register my account, it says I've registered too many. Is there a way to delete the past usernames? I know I have three called TheRunningArtist, Keys0, and PhotographyKeyArtist(yes, I know, pretty weird usernames).