Oooooo I can understand your frustration! Gross!! My younger brother did the same until he was 11, coming up close to his 12th Birthday and my parents invented quite an elaborate story and convinced him that after 12 years of picking your nose and eating it, your nose caves in and your belly is so full of snot it won't work anymore so you have to have an operation on your 12th Birthday to suck out all the snot, but there's no way to fix the nose, it's caved in forever, I think they even found horrific photos of damaged noses etc and told him they were child nose pickers once upon a time and they didn't stop before they turned 12! They told him the only way to avoid the operation and having many needles and a caved nose forever was to stop picking your nose the day before your 12th birthday... Now I look back, WOW what a HORRIBLE way to influence the child, but IT WORKED! Well, that we know of, we never saw him pick his nose again, he always carried tissues and still does to this day... The story terrified him! Maybe there's something a bit milder than this method you could try if you're not feeling as mean and as desperate as my parents were that day!
lol Good luck!!