www.abc.comwill not block the site which has URL
https://abc.com. If you add the URL as
abc.comthis will also block all URLs matching
yyyabc.com, so the best way would be to use
.abc.comformat when adding URLs.
Due to browser security particularities limitation by website URL work only if you have secure connection enabled in your account. So be sure to enable the secure connection option at Account Settings / Connection page in your Control Panel.
You can also limit access to your chat from certain countries or block access by IP address.