When you send to "All" you are sending email to everyone in the global address list. You can hide a user from the GAL by doing the following:
- From Active Directory Users and Computers
- Go to View and make sure Advanced Features is checked
- Now find the user that you want to hide from the address list and select properties
- Click on the Exchange Advanced tab
- Check "Hide from Exchange Address lists"
- Click OK
This will hide the user in the GAL, it will however stop the user showing up in the address Global Address List when users are looking at it to send them a mail, which i'm not sure if it what you want.
If you want the user to still appear in the GAL for lookups, but not recieve mail sent to the GAL, then you have a bit of a problem, as they are kind of contradictory.