Employees work from the Horse Management page ("horses" tab -> "manage"). From there, select "care" and you should see a check box at the top that says "groom" if you hired Davita. You will also see check boxes for "feed" and "water" if you hired the other employees. When you check the box it will auto-check every horse you own so you can groom (or feed/water) all of your horses with just one click.

Not sure about removing employees and any money back, maybe someone else can answer that part of your question.

Okay thanks!