Lots of good advice here, glad some of it has been useful, OP! I've not experienced this exact same thing; but a couple times I've been to someone's house and they were a bit pushier about offering me food or drink than I was expecting. I think they really felt like they must have been doing something wrong as a host, because I wasn't eating/drinking something
right then, so they kept saying, "Do you want A? Do you want B? How about C?" Each time I would say, "No thanks, I'm fine," and at about the third time I would kind of give them a little look before replying, like, "Have I started speaking a foreign language here?" That usually gets it across.
The other thing is my grandma is a "feeder," but only of men.

I remember once my parents and I were having lunch at her house, and she looks over at my dad (who is her son-
in-law!) and says, "What else can I get you to eat?" My dad was like, "I've got a sandwich in one hand, fruit in the other, and a plate full of food in front of me. I'm
fine." Meanwhile, my mom and I are hopping up to get stuff for ourselves
and Grandma. As she's gotten older and less physically able this has diminished somewhat, though.