I've not had that problem myself but a few things that might help:
1) Shoot them so the face in only slightly turned from the camera. You don't want to shoot the head on but the more they turn their head the more obvious the size of the conk will be especially once it starts to break the outline of the face.
2) Use a longer focal length to compress the perspective.
3) If you're using lights have the key light only slightly off axis, the more off axis it is the more of a shadow it will cast across the face and the bigger the nose will appear. You could also have the person face towards the light, if the nose is pointing directly at the light then it won't cast a shadow, but the lighting might appear a bit flat,fine tune it to suit.
4) Alternatively do exactly the opposite of everything I've suggested and make a feature of the nose, the sitter will know they've got a big nose so unless they're particularly conscious of it then make the most of it.