I always started by teaching my kids that when they bring their friends home they get water.. If they should by chance offer juice or pop it was not bought again for them for a loooooooong time. My daughter made that mistake my son has not yet.
Man, some of you guys are super protective of your drinks. Whenever I had friends over, my parents always expected me to offer the guest with something to drink from the fridge. It's basic hospitality to me.