Well, first of all you can't tell them no. It's their business and they do have the right to ask you to leave or put it in your car. Please don't make the mistake of arguing with them or doing something illegal. Once they ask you to take it out, you can't tell them no and drive on. You could risk getting charged with trespassing, along with whatever other charges LEOs want to tack on.
One of the things you can do is complain to corporate offices about your treatment. Another is to make sure you never support a business like that again by finding more accommodating merchants.
Don't risk your possible future career by doing something illegal like saying no. There's other ways to deal with situations like this