I know that at my wedding I said "Thank you to everyone for all the gifts" I didn't go around to everyone and say thanks a hundred times nor did I send thank notes. I just didn't have the time, I was preggo plus being a new wife and then having her I was very busy and if someone got mad b/c they didn't think I thanked them "properly" than they can bite me. I don't care if they send me another gift either b/c that just means they have no understanding of situations like that anyway.
but if the op knows that this girl hasn't said thank you in the past than why get mad when she doesn't one time? if she is so worried about stop it giving the gifts if all you want in return is a thank you. or better yet call her and talk to her. how hard is that???
eta: personally if I had given a gift that I didn't get a thank you for I don't think I would have even noticed..