As a former nice person, I know it’s hard for nice people to say no. But it must be done. Especially to insistent jerks. LOL!
I have also perfected a look of disdain to accompany my second no. The third time I break out my megaphone, get two inches from their face and blast their hair back with my reply (at least in my mind.)
And I agree with everyone else. No fork sharing, except with loved ones.