Say jokingly, “No thanks, I’m watching my girlish figure.” This is especially funny if you’re a 6’ 3", 250 pound dude with a goatee.
Just say, “Thanks, but I’m on a special nutrition program right now to get healthier and can’t stray from it as much as I’d like to.”
Don’t say, “I’m on a liquid diet.” Say, “Nutrition program.” And say “to get healthy” not “lose weight.” Even granny isn’t going to discourage you from being healthy, right?
Reminds me of what Dave Tate wrote in his new T-Nation article:
“The other thing I suggest when setting goals is to keep your goal specific to yourself and a select few, and general to all others… Your goal may be to get your bodyfat down to 6%, but all the masses need to know is, ‘I’m dieting right now.’ The reason for this is simple: 90% of everyone you meet are negative pricks who will go out of their way to tell you why you can’t do something. Once they know your goal, they’ll try and tear you down. Just keep it vague, and all they can do is wish you success.”