I am not aware of any criminal liability a bartender may have for serving someone with a B card. And in Minnesota, you can legally refuse service to almost anyone, and can most certainly refuse service to an individual you know has a B card. So, I don't see any civil liability for refusing to serve someone who has a B card.
On the other hand, you may risk civil dram shop liability to your establishment if you serve someone you know has a B card, and then that patron goes on to cause harm to himself or others. You should follow your employer's policies regarding this issue, and if you are the business owner you should consult with an attorney to determine how to protect your license and your business.
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