Sharing in Galaxkey works on group level. If you want to disable sharing a file with a particular user just take out the user from the group by going though following steps
1. Right click on Galaxkey for Windows tray icon and click on "Groups" option.
2. Select the group from which the user needs to be removed and click "Modify" button.
3. Select the email address from the "Current Member List" and click "Remove" button.
4. Save the changes by pressing "OK" button.
The files in the associated group folder will no longer be shared with the removed user.