Conditional Fields is a great Drupal module for conditionally hiding CCK fields.
Unfortunately, it can't hide CCK fieldgroups. It's also not ideal if you're concerned about security— it simply hides fields; it doesn't deny access. A recent project of mine required that I conditionally deny access to field groups, so I decided to implement a programmatic solution.
In this case, I needed to deny access to specific fieldgroups in content type 'company' based on: