Other than persistently trying to get her to use a tissue we've told her that we don't want to see her with her finger in her nose and if she wants to do that she has to go and do it in private in her room (not that she ever does).

We were having breakfast this morning and Grace had finished and was sitting at her table. She was facing away from us but clearly had her finger up her nose. "Grace, take your finger out of your nose." To which she protested, "I'm facing the other way!" (i.e. but you can't see me).