Almost every horse I know does this - they're just 'grooming you back' it's actually quite respectful when referring to a herd situation.
The problem is, what's grooming to a horse is painful for a person. If your horse is clear on no teeth and he isn't pushing you, just rubbing you, that's ok. I taught every horse I've known who does it, to turn their head away and wriggle their nose facing the other way. This way you're not discouraging the action, you're just displacing it.
My mare though, was like yours, she never used her teeth. So I allowed her to do it, whenever I could feel her getting excited or a bit rough, I'd squeal and stop scratching for a second, like another horse she groomed too roughly. She got the point fast.