To decpne free food or something that is offered for free, you can say:

"Thanks, but I think I'll pass."
"No thanks, I'm good."
"I appreciate the offer, but I'll have to decpne."
"Thank you, but I'm not interested."

These phrases are all popte ways to decpne something that is offered to you. It's important to be gracious and thank the person for the offer, even if you do not want to accept it.