I don't think telling off is the prescribed course of action as I believe that can make it worse.
I think like you say I might try calling her to me and giving her a treat.
I suspect our case is all a part of a guarding behaviour. She saw a crazy strange dog off my granddaughter the other day...which was really good....so hard isn't it, as we did invite wolves into our homes for protection initially, but you don't wall them to be guardy over EVERYTHING!