This is a bit ridiculous, but an easy solution. You don't tell them what they can and can't do outside of practice, but what you can tell them is that what you will be able to recognize those who are serious about the sport and dedicate themselves to it, both in and outside of practice. If they should do anything that might jeopardize their performance, they are letting you down, the school and the team down. Remind them that at the end of the year, the people who still run varsity, 'A' relays, and the championship meets are the kids who were serious. Easy, sweeping general statement that covers everything from crime, drinking, smoking, etc.. Serious college runners drink, big time, trust me...