"The correct thing to do, even if you don't like it, is to take it, say 'Thank you very much', and then either eat it with a smile on your face, or say you'll keep it for later."
I agree with this answer which was in the comments.
At the risk of sounding racist there is a saying: "Do not accept gifts from Greeks".
"Beware of Greeks bearing gifts"
http://en.wikipedia.org/wiki/Timeo_Danaos_et_dona_ferentes
There are some contexts in which it is inappropriate to accept gifts. In that case you would probably say something like:
"I am very sorry but I cannot accept your gift" or "I am very sorry but we are not allowed to accept gifts".
There is also a problem when something is given as a substitute for a payment. If you have agreed to teach English for a couple of hours you might but unhappy to receive a bunch of flowers for your troubles.
There is an interesting expression:
Cannot be had for love nor money.
http://idioms.thefreedictionary.com/for+love+nor+money