You can do this http://www.cisco.com/warp/public/788/AVVID/unity-filter-vm.html.
Or you can give them a second mailbox to use only for voicemail. You can hide the second mailbox from the GAL. If you still want users to have access to the Active Assistant you can do the following:
If you're using Exchange 5.5 make the users NT account the owner of both of the mailboxes.
If you're using Exchange 2000 you can use grantunityaccess.exe to associate the users NT account with their Unity subscriber account.
Keith