It is something they learn without having to be taught I think. Like eating the cream out of the middle of oreos (except that is waaaaay cuter)
Ella does it too and my magnificent way of handling it is usually saying something to the tune of "No Ella! Not on your sleeve! Get a tissue!"
Very effective, she freezes mid swipe and then obediently trots off to get a tissue
She does know how to blow her nose though, so it lasts a bit longer...
Sorry, this was totally useless response wasn't it?
Hope you get some good ideas!