That line of the Wiccan Rede is frequently misunderstood. There is no way we can go through life without causing harm to someone or something.
The line reads, "An it harm none, do as ye will." In this case "An" should be interpreted as meaning "If". So IF an action will not cause harm, then you are free to pursue it.
The Rede is silent on those actions which will, or which might, cause harm. In those cases it is up to the individual Wiccan to decide the appropriate cause of action. Sometimes harm is inevitable or even appropriate. Sometimes we need to reassess our options and choose a way that will at least cause less harm.
The Rede is a guideline to help us look at our choices of action and choose that which is most ethical in a given situation. It isn't a simple rule to live by, and it lays all of the responsibility for our actions squarely on ourselves.