I've had MinPins for years and what I do with mine is a three bark rule, and then I simply thank them for alerting me to the big danger out there and redirect them to something else, sit, down etc. Make it into a game and a job they have done for you. MinPins are notorious for barking and being little guard dogs, so reward your dog for doing so, only three barks! It takes time and work but it can be done.
When the pizza guy comes to my house, they all have to run and sit on the mat, thus they then get a treat, same if someone comes to the door. They are all little piggies and happy to work for treats. Make your little one work. You will be shocked at the difference, water sprays etc is nothing to the stubborn little *******s, BUT being as smart as they are, make it a game and fun, you will love the change in your dog.
MinPins are traditionally a working breed, make his little butt work!!!!