I already put my spoke in, but something Rob said made it necessary to chime in again.
If we tolerate bad behavior, it will only get worse. When someone does something wrong, let them know, in no uncertain terms. Be polite. Be civil. Be stern. No matter how angry you may be, keep your emotions under control, but let them see that you are doing so. Some of the neighborhood kids are real hellions—but they are always respectful (for them) and obedient to me—I may yell to get their attention, but after that I speak quietly but sternly to them—end of problem. When you can do this, you stay in control and cannot be the bad guy.