The secret: don't take it personally. It's not about you. It's about their bad day. Take a breath. Stay calm. Apologize for the inconvenience. Offer to remake the drink. And if they're still rude, ask them to leave. You don't need that energy in your café.
The Service Golden Rule:
I have one rule for handling difficult customers: treat them with dignity. You can be firm, but you must be respectful. You can say no, but you must do it politely. You can refuse service, but you must do it without anger.
Because the minute you lose your temper, you've lost the argument. The customer will leave, tell their friends, and post a negative review. And you'll be the one who looks bad.
So stay calm. Breathe. Smile. And remember: the customer is not always right, but the customer is always the customer.