As far as I know they can't make you stop carrying. They can ban you from the restaurant if they find out but if it's concealed it's concealed and I don't believe a company can make it illegal to carry a gun on their premises
This is worth checking out the local/state law. In many states, the business owner (or his representative) can ask you to leave, and if you refuse you can be charged with trespassing, but otherwise there's nothing inherently illegal about carrying in a private business where guns are prohibited.
But, I believe there are a few places where this is not the case, and simply carrying inside a private establishment where it's prohibited is actually illegal.