>>2441550
1. get live trap
2. trap and keep a large number of mice/rats
2.A. alternatively, buy mice/rats/etc, but make sure they're not "fancy rats" or "fancy mice," because those are readily identifiable as pet rodents
4. figure out a covert way to introduce mice into the bar, could be as simple as putting them in a cardboard box in the alley behind the bar
5. go to the bar, get a picture of the mice in the bar
6. anonymously send it to the health inspector
7. there is no step 7
8. profit
also be advised that a possibility exists where your building also gets full of mice
other pests could be used in place of mice/rats, but critters like roaches can be impossible to remove in some climates and are likely to spread to surrounding homes/businesses.
I had a friend in high school who kept trying to get fired from Tim Hortons. His mum had some kind of an agreement with the manager/franchise owner, so no matter how awful an employee he was, they retained him all the while his mum wouldn't let him quit. So eventually he caught a bunch of mice and let them go in the building, took some pictures and informed the health inspector. That Tim's shut down about a week later and remained closed for months. His mum told him not to work there because it was "filthy."