This past year we had to demand a young person who had rented a some outside stalls from us on a self-service situation to leave our farm due to her maltreatment of her animals. She did not feed, water, exercise, turn-out or provide medical and farrier on a regular basis.
A few years ago I also had to ask a boarder to leave because her mother would come in her high heels and skimpy clothes (were talking lingerie sometimes and once she wore red higheel shoes with feathers on them) to bring her horse in from the park and knew nothing managing a group of horses all wanting to come in together. I was a dangerous situation and luckily we had no injuries.
We asked another to leave because she would leave her kids (8-12) here all day with no food and after 2 hours they were done with their riding and we became unwilling babysitters. That resulted in our no-kids policy which has worked out great for us and our boarders.
I am curious about other BO's reasons for asking Boarders to vacate.