Hello,
If the account is disable in Active Directory they will not be able to access the system using there diabled AD Credentials.
However, SharePoint doesn't remove the users from people picker on the site.
While searching user is people picker, very first place the SharePoint check for the user account is in user information list.
If the user is in the user information list, then the user information is returned. If the user is not in the user information list, SharePoint will check the user from AD.
Because the user was added to the site before disabled from AD, you can see him in the people picker, you could delete the user from the site collection to remove him from displayed in people picker.
At first, check if you are getting the disabled account in user profiles in SSP. If its not there, then surely you need to remove users from site collection level.
Regards | Rohan Chittyal | Sharepoint Technologies | http://sharepoint-administration-eu.blogspot.com/