Deny access to specific user(s) in a site collection.

Hi, I was wondering if anyone had idea how to accomplish denying a small number of users from accessing my companies  intranet without changing any AD groups. Right now the intranet has got the AD group Domain users in the Visitors group, so all domain users can access it. Is there a way within SharePoint to deny access to a small number of users within the AD Domain user group. This has been requested because we have two people who are leaving and they want to maintain their AD groups, as they will still be doing some work for the company, but do not want them to access the intranet, and I have been specifically asked to accomplish this within SharePoint without messing around with their ADS accounts.

Any help would be appreciated.