Call 'em up and place a large carry out order.
Show up to get it. Notice the no guns sign. Knock on door and ask to see manager. Explain why you can't come in to pay for the order. Leave.
That'll cost 'em something.
Now several folks will post and say that this isn't the High Road.
Wrong. It is HIGHLY effective, and it's a darn sight more "High Road" than the "cold dead fingers" approach.
They are selling something. If you order, and then are not allowed into the store, hey, it is NOT your fault. Make sure that when you show up to pick it up that you do NOT look like a lowlife. Not saying "suit and tie," but think "business casual."
Oh, and you don't want nasty surprises either - what do you do with your "large order" if you show up and they've already taken the sign down? Just buy dinner, and then take the family somewhere else. Send the pizza joint a copy of the receipt. "Dear Sir/Ma'am, I'm sorry I can't buy pizza for your store, but until you remove the no-concealed-carry signs, I'm having to go somewhere else. I really miss the quality of your product, but when you give preference to criminals over people who can pass all the background checks to get a firearms license, I just don't feel safe in the area of your store. Sincerely, Joe Gunguy"
Their marketing folks assume that for everyone who complains, there are 6-10 folks who just go on... And it COSTS to gain a customer, and they lose that money when they lose you.