Hi K.,
My daugther is 25 months and has been eating out since she was 3 weeks old (okay, so she didn't EAT at that age, but I nursed her in restaurants!)... :)
I remove my daugther and end our outing at any time that she does not sit in her chair, eat her own food off her own plate and mind her manners. She knows that these outings are treats and that if she does not behave, she will not get to participate. Also, I bring her own cup, plate, utensils, etc. so that it's comforting for her to see things from home and I always carry small books, crayons, coloring books, etc. so that if she has to wait at all, we are occupied. Also, I've gotten in the habit of ordering for her first, as soon as we sit down and make sure she has food when she needs it (even if I have to bring my own), b/c I know that some of the people we go out with or even the servers, are not tuned in to how hard it is for a toddler to wait for their lunch, dinner, etc. at a certain time of day.
Honestly, though, I think she just needs to understand that the behavior that you don't want her to use will not be tolerated. I know it's also going to be hard on you and your husband, but it means one of you taking her to the car and explaining the situation, while the other one boxes up the food in to-go boxes to take home. I guarantee that it will work if you are consistent and do it every single time. You may also try taking her outside the first time, talking it out and seeing if she will change her ways if you bring her in again. If she doesn't, though, you have to be ready to leave. The more you give in, the more it will happen. And the fewer chances you give her, the quicker the message will get through. It's called, "Striking while the iron is cold" and read John Rosemond if you want any other great discipline tactics! :) He's the BEST!
Good luck!
K.