Yes, you could make it clear that you didn't want the neighborhood watch on your property but was that the case here?? There's nothing to say that it was. You're introducing a variable that no evidence of exists. That would be like a Zimmerman defender saying "Well maybe Martin picked up an axe that soneone left outside of their house....". There's just no evidence of it. But even if it was, that would make Zimmerman guilty of trespassing, not murder. We have to deal with what we know....not with unsubstantiated "maybes" and "what-ifs" of which there are an infinite number.
I've lived in gated communities. Obviously the neighborhood watch was sanctioned by the HOA. It wouldn't have been operating inside a gated community if it wasn't and the stories I've read make it clear that the watch had been operating for a while. And if Zimmerman were a legal resident of the community (quite likely since gated communities don't usually allow non-residents access like that) then he could simply do it on his own.
My development has a HOA and although we're not gated, we still have "common ground" such as a swimming pool, clubhouse, and rec area that's owned and operated by the HOA (of which I, as a homeowner, am a member). When I bought my house I bought into this common ground and my monthly dues maintain it. It's considered private property of which I am an owner. I would be 100% within my rights (and anyone else whose legal residence is within the development) to confront someone on that property whom I thought was trespassing or up to no good. It doesn't give me the right to harass anyone but I can question them and expect them to respond accordingly just as if they were on my own personal property.
Zimmerman may have broken more than one law but confronting a stranger within this gated community doesn't appear to be one of them. He was well within his rights to follow and question an unrecognized person who he believed was acting suspiciously. It's what happened after that that'll determine who's at fault.