Thanks for the reminder to check the archives! Helpful stuff there :) I also found this:
"Marty used to eat boogers when he was little. Four, five? We had a friend who worked at the zoo, and I someitmes offered to ask Bill to bring us some exotic boogers Marty could try. It was a way for me to say it was gross without ragging on and on about Marty's own boogers. I would say "I wonder if they taste like monkey boogers," or "I wonder how big an elephant booger is." Just conversationally, like I was talking about anything else. It was kind of funny, but kind of gross, and it probably helped him not do it.
I also said to each of my kids at some point when they were pulling boogers out or eating with their mouths open or whatever, that there are holes in our bodies, and there's stuff in there, and it's not nice to show any of that to other people. Wiping boogers on the wall is something like rubbing poo on the wall—something that should be inside us, or cleaned out privately.
MAYBE (depending on personalities) you could put little marks on a piece of paper—not the calendar on the wall, but just a little chart that can be thrown away easily, and put a line when you see him with his finger in his nose. One day might have six marks. The next might have eight, or only two. There might come a day when there are no marks. It might help him AND you to see progress that way.
Tic marks, like 1, 1, 1, 1 and cross it to make five. "
I think my nine year old would really like a chart like that, especially if I could mark on it without necessarily having to say anything in front of the other kids. She said she doesn't even notice when she's thumb sucking, but if she thinks about then it's easy to stop.