MikroTik Proxy (webproxy-test package) when you install it you can find on the menu
If you want to deny access to some page:
/ ip proxy access add dst-host=www.msn.com action=deny
And if you want to deny some page with redirect to your one polici page
/ ip proxy access add dst-host=www.msn.com action=deny redirect-to=www.yoursiterulesandrights.com