I would help them advertise their noble decision to ban guns from the premises. I would write all the local gun clubs, ranges, and sporting goods stores in the area, and attach a picture of their front door in the e-mails. I know I would appreciate someone letting me know of their policy before I wasted a drop of gasoline driving there.
Do so on Social Media as well.
The days of "Any pub is good pub." are over.
If even 5% of their customers are actively pro-gun, they definitely do not want to take a 5% hit on their business volume.
NoGuns=NoMoney cards have worked well around here.
Don't Shrug. Act.