You can modify your cursor sets by clicking an "Edit" link on the page with the list of cursor sets (http://www.rw-designer.com/cursor-library/set-70) and replace an old cursor with an updated (new) version.
You can also hide individual cursors that are not in the sets, but you cannot delete them. This is for security reasons - the access to your account is not encrypted and in case someone discovered your password, they cannot delete your cursors.
I occasionally delete hidden or duplicate cursors, you need not worry about it.