None of my kids did that, but I think if I ran into the problem I would have started by telling them that it isn't something people do (or people their age should be doing) in public, and telling them if they were going to do it to please go in the bathroom for privacy. I don't know if this is what experts would recommend parents do, but my approach would be to first try to "drive it underground". Why? First, to see if the child could actually control the behavior enough to learn to do it in private, but second (and I'd explain this to the child) to stop what is considered socially unacceptable behavior and stop other people (including classmates and friends) from finding him objectionable or making fun of him.
Having been a thumb-sucker and having been "driven underground" by a father would kept "at me" about it, I know that a kid is likely to think, 'It's my thumb, and I'm not hurting anyone," or "It's my nose...". So I suppose as a parent I'd add something like, "I know it's your nose, and you can do what you want with it; but there are some things people just don't do in front of other people." From "underground" came phasing out of the habit on its own.
Of course, I'd also explain (in a calm, neutral, way) that "the nose thing" means getting germs on one's hands, and germs can make other people sick (especially babies and older people). I'd want my child to know there are solid reasons the behavior is considered socially unacceptable. I'd also add that he should wash his hands really, really, well if "touched" the inside of his nose or "got germs" on fingers.
Having said that, the behavior is common in children. Most outgrow it, and sometimes it's nothing more than a really nasty habit. Other times, however, it can be associated with psychological disorders, including, possible, OCD. This is one reason I might aim to see if I could "drive the behavior underground" as a way of getting a reading on how under-control it was for the child. It's really important to stress, though, that it's a REALLY common thing for kids to do. Most of us have seen more than one kid (or even adult) who has done it.
One final thing: I'd try to help a child know that this is something a lot of kids do because "they're kids and they're not finished growing yet"; and even though it was something he needed to learn to stop doing, the fact that he does it doesn't mean there's "something wrong with him".