want to remove a website that has been replaced with a new one.
Remove it from your server? If so, you’d need to use an FTP program (Yummy, Forklift, etc) or possibly the control panel/file manager of your host and delete the files that way.
If the URL’s are different (which they presumably are if you’re having this issue), you also probably want to consider redirects for the old URL’s. Google would be a better source to ask than myself on that one, though.