The ZAP- essential, but not sufficient
I see the Zero Aggression Principle as essential for ethical behavior, but not sufficient. I "really believe" in zero aggression, but I recognize it is a part of ethical behavior, not the whole enchilada. In other words, you could never initiate force, and still be a bad guy.
Trespassing and theft (including fraud) may not involve any force at all, and many would conclude that they do not violate the ZAP in that case, but you and I both know that either way, these acts are wrong to engage in.
I recently read an anarchist arguing that shoplifting from a giant corporation is good because (and I paraphrase) "who owns the box of noodles?" All I can know is that I know it would be wrong for me to live that way or to act upon that kind of belief.