Please put the CSS code in Top Black Admin Bar > Customize > Additional CSS, delete all caches and press Ctrl+F5 (twice) on that page. It should work fine.

#wpf-msg-box {
   display: none !important;
}

I have all the right controls Disabled, as per closed topic info, but still have one problem not mentioned in topic or elsewhere.

I want forum to be for members only, and all members are registered by website MemberPress system. The forum is an add-on for website.

I want non-member "Guests" to be able to read forums, but not post, and not sign up. All Ok so far.

Problem is that a guest can browse topics Ok, but when he clicks a "Comment" button, a Login/Register POPUP appears at top of screen. Guest can click links and be sent to a suitable page, BUT do not want the POPUP to appear at all.

How do I disable this?

Thanks. See image for popup in question.