OH NO !
I dont know how to stop it - sorry ! I have caught my 6 year old picking his nose a few times and shot him down with "the look" - thankfully i have never seen him eat it.
My Niece used to eat her boogies and i was always completely grossed out - i had to say something - i couldnt help myself- yukko. They live on the mainland now so i dont have to see it.
Kids like gross things - maybe they like the warm gooey sensation !
Hard one - it definitely becomes a habit !

sorry, couldnt help myself - some of the comments had me in fits of laughter.