I'm not going to say thank-you the next time my boyfriend buys me some cakes
Whoa! Why don't you want to say thank-you?
I hate sweet stuff and I'm going to try to lose a lot of weight because I want a flat stomach.
It's okay that you want to cut back on sugar in your diet, but you should tell your boyfriend this so that he doesn't try to encourage you by buying cake. It's wrong not to say thank you when someone buys you a gift, and it could hurt your boyfriend's feelings if you wait until after he does so to tell him that it was unwanted.
He should stop keep buying me cakes.
Yes, but he doesn't know that it's against your wishes. You have to say thank-you, but at the same time covey your preference. He's only trying to be sweet.