Yes, technically, it is trespassing, since to remain in *any* location after those with the right to control access or use tell you to leave is trespassing, and so yes, you could contact the police (which is how you deal with trespassing). Of course, calling the police on your customers is  very good way to go out of business very quickly. This is a case where enforcing your legal rights may be a very bad idea. A better idea might be to, when new customers sign up or existing customers renew their contracts, to add a new clause or provision to the contract that customers who remain after closing 3 or more times will be charged an extra $10 for each time, after the 2nd time, they stay past closing (or something similar to this). This may discourage this behavior; or if not, will at least bring in extra revenue.