How can I tell this girl I like that I don't want to hold her lunch?

She did me a favor once and now she keeps telling me if I can hold her lunch in my gym bag until lunch time so she doesn't have to carry it around and it STINKS, I'm sick of her food havin my bag smellin like a ghetto barbecue, how do I break it to her, in a nice polite manor. I'm not good at being nice.