How would I delete a domain?

Please use the search before posting.

You cannot delete a hosting account. Just delete all the files from it and ignore it.

What if some folders weren't deleting?

Ultimately it doesn't matter.

But it is probably because they contain hidden files such as .htaccess files.

In your FTP client you need to 'force show hidden files' and then you will be able to delete the directories.

Excellent, it worked thanks.