While it is indeed bad form to arrive at a restaurant with your own food and booze,perhaps putting a little cardboard tent on the tables with the restaurant policies (1 drink min/
No outside alcohol or food.) Would be a more,effective and professional way to get the message out consistently, and without offending your customers. Your shaming treatment of people (even those who just stop to say hello and have no intention of staying) has not gone unnoticed. This is still Mexico, and rules surrounding this issue have been very relaxed, and in many places they still are. It is not good form to be rude to customers.
I would be careful, however, about shoving the menu board on people or demanding that people are required buy food with their drink. While Ricardo and other musicians may draw the people in, believe me, the food wont keep them there.

Best of luck.