^ He could have taken it off, it's just that we the audience wouldn't have seen it. They could do it a bit tongue-in-cheek, like an Austin Powers movie, with a series of things covering Dredd's face (as oppose to Austin's genitalia) or simply pan the camera away and film him from behind when he takes it off.
Yep - there have been scenes in the comics when he's taken his helmet off, and we've only seen other characters' reactions, or him in disguise with a temporarily false face, or - in at least a couple of memorable occasions - with a "Censored" bar blocking out our view of his face!
But we've also see him in the bath with his helmet on, and even in a sleep machine still wearing it!
Of course for multiple issues of 200AD Dredd didn't wear his helmet and his face was on show the whole time
