The item you are trying to hide isn't part of the application and doesn't really fall within the realms of the support, so a tad unfair to make that comment.
Anyway....
Add this to your custom.css (may or may not work in all browsers - if necessary, there is a plugin method that can be used to hide the field)
li! > span > input[name="reg_admin_mails_checkbox"]{
display:none;
}
Warning: as you are hiding this field, you should set the following to 'Opt in' so that the hidden field is unchecked by default, and you aren't forcing them to sign up for something without knowing etc etc.